This is a complicated question. For starters which Sora, Riku, and Mickey are you talking about? Like if we go with KH1 version of Sora and Riku then they are bottom barrel. If we go with DDD/KH3 versions then they are near the top. At the same time how do we quantify strength because if we are talking Magic Aqua is easily at the top alongside Sora and Mickey. But brute physical strength Terra would be at the top with Riku next and Mickey is at the absolute bottom given he's got the build of a stuffed animal. Then we have individual strengths like in terms of raw darkness power Terra and Riku have the most but Terra has absolutely no aptitude for controlling it. Meanwhile no one quite has the same level of mimicry learning that Sora demonstrates or the silly fluid adaptability that makes him come up with lots of creative ways to use the power of his friends.
Mickey is canonically the strongest or one of the strongest present day. He was already near/at master level back in BBS putting him around Terra and Aqua's level, with years of training under his belt. AND unlike the rest he's been fighting in every crisis since then including battling in the realm of darkness for awhile. Mickey has shown immensely powerful magic and incredible fighting skill, his downsides are that he again has the build of a stuffed animal so he can be neutralized pretty easily. Mickey only needed to use a portion of his power to complete suppress the shadow of Ansem, he destroyed a few darksides in a single blast, he froze the entire organization with a stopza, and he's done really wild things like use the power of the keyblade to extract the core of Data Sora's heartless immobilizing the main body while Sora finished off the tiny shadow. When teamed up with Aqua and Riku he pulls off all kinds of neat light based attacks that eat things alive. Hell even his partner card in CoM is OP he hits the entire field with a wave of golden light that deal damage, heals Riku, stuns all the enemies, and reloads his deck. Even in Days Mickey is one of the strongest and most agile playable characters.
For this reason the game basically has to constantly come up with ways to keep Mickey out of the plot post KH1. Be it separating him from Riku for an extended amount of time then Riku requesting Mickey wait while he fights Ansem alone. Having Mickey being stealthy, constantly running of on his own, and Xemnas closing the door and later corridor on him in KH2. Mickey not realizing that he was in the computer in Coded until later, Data Sora telling him to stay back while he handles things, Mickey getting lost in Hollow Bastion somehow, and then Mickey not making it to CO until the end of the game. Finally we have DDD where Mickey physically can't get to where Sora and Riku are for 90% of it and 0.2 where Mickey shows up only at the tail end. The constantly excuses to keep Mickey out of big fights becomes more transparent by the day.
So ignoring the stuffed animal property weakness he has Mickey is top after that we have to look at skills as a keyblader. Aqua, Terra, and Ventus all went through formal training so they start off with more skill. Riku and Sora were self taught by using the world around them to teach them and getting other kinds of training from variou sources. Still it's not until DDD/Post-DDD (after they apparently were learning how to use the keyblade properly) that Sora and Riku started to realize the full capabilities of their keyblades. Be it for non combat purposes like using it to re-write hearts or combat purposes like turning them into yo yo's of death So there WAS a gap there where TAV were ahead of the game but that gap isn't really there anymore. This is perhaps best highlight by how Riku takes down both the closest thing we've ever gotten to MX prime (his power, mind, and skill but in the untrained body of his young self fresh off the islands) and a stronger version of Ventus. In KH2 Sora is able to best a Terra with 11 years more rage filled will empowering him, but he does this with the help of Donald and Goofy. In DDD however Sora is on equal footing with Xemnas who is canonically one of the strongest characters in the series, where just one game ago it took Sora, Donald, Goofy, and Riku fighting through multiple phases to take him down. (Yes he had the power of KH backing him up but it's clear while it did make his a fair bit more powerful, that when you look at his other fights in KH1 and DDD it wasn't AS huge an increase, he still had the power to throw multiple buildings at Sora without KH powering him for example). Point is Sora and Riku started near the bottom but they've come a LONG way in terms of being wielders. In contrast only Aqua has been allowed to continue to grow in strength though her time in the RoD was much shorter from her perspective than we were ultimately lead to believe. Terra and Ventus were deliberately forced into stagnation though Terra was already near master level, Ventus was just figuring things out. However they might of passively been growing in strength via Ansem/Xemnas/Roxas so eh....again it's hard to quantify this because we only have one game to show us what they can do and what they do isn't super impressive. However stronger versions of them super charged in various ways, like Ven's will charged by the infinite darkness of the abyss of sleep, were able to be quite impressive.
In terms of physical skills Terra is near the top. The rest of our wielders have magic keyblades to give them super strength they don't normally possess, as shown by how Sora could barely dent a heartless in KH1 when he lost his keyblade....yes it was a wooden sword but a wooden sword can still hurt and his magic was a lot weaker as well. Terra is a very burly man he even does handstand push ups in his break time command. Riku is much the same look at his body he's a guy who looks like he pumps some iron and this is further supported by the fact that Riku went through almost two entire journeys relying on NOTHING but his physical strength. He didn't have any real dark powers for most of KH1 which is why he can do jack all to Parasite cage. Then in CoM while for gameplay purposes we can choose to use his dark powers, canonically he refused to use the power of darkness till DI. Meaning he beat up Dragon Maleficent, Hades, Hook, etc... and all those heartless with little more than his brute physical strength. Physical strength becomes a little less important when you're talking keybladers since it is powered by your heart but physical strength DOES still translate. Aqua given her history would probably come right after them with Sora and Ventus straddling the bottom, in fact given the way they implement Ventus as a character he pretty much would be bottom. However along with this strength we have to look at fighting style Terra for all his raw strength is very slow, he hits like a truck and has good form but in the end the sludge style of his drops him some. It's not a surprise Xemnas would fight equally sludgy given he's Terra's body and we've seen Xemnas's own melee strikes aren't actually that hard to deal with it which is why he has to mix in other things to get those openings. Aqua and Sora are firmly of average speed, well Sora does tend to actually show some amazing speed here and there, and strength with Aqua being a little bit weaker while Sora is more of a jack of all trades. Ventus is very weak but he is easily the fastest and he uses his speed to make up for his lack of strength, a lot like Larxene, I wouldn't call Larxene a weakling so I won't call Ventus one either. His speed is a good equalizer putting him right up there with Aqua and Sora. But then we get to Riku and this guy has it all from fantastic strength to great speed, he's a jack of all trades and master of all. Riku wins.
After that we move into category three magic. If we go by just all kinds of magic in general it's Aqua by a landslide she may not have shown in-gameplay quite the same level of magical battle skill as Sora and Riku, but she HAS shown that she knows all kinds of magic beyond just battle magic from making the magic charms that saved their bacon to the spell on Kairi's pendant that saved her life. I think it's safe to say when it comes to magic she is the ice skate queen. BBS doesn't do her power justice or maybe it does and 0.2 is more a result of how powerful Aqua had become BECAUSE of her fighting she did in BBS afterall 0.2 doesn't start you at level 1. After that we move to Sora who has grown a lot as a mage from his raw unfocused power in KH1, his ability to perform most of the more impressive complex magic like holy/quake/warp/etc.. in CoM, his ability to manipulate his magic to do more impressive things like making fire wrap around his body or stuff like dancing thundaga master form. Finally in DDD his magic seemed on par with BBS TAV if not a little above it, he even had his first -Za level spell. I'd put his firm just below Aqua in this department after that it gets a bit murky. Terra can use magic pretty well but his scope of spells is much more limited and there is a heavy emphasis on pure power without the finesse. Still his raw power in this department shouldn't be underestimated but in this case I think we can put him near the bottom. Riku would be right there with him because he had jack all for magical skill that wasn't darkness based until DDD but he was pretty much tied with Sora and the rest at this point, true to his nature I'd put Sora and Riku as equal levels both just below Aqua but above Terra. Ventus...I'm not sure what to say he feels like the ultimate middle ground having powerful magic but not showing the same level of aptitude for it unless it's light magic. Given he hasn't been given a whole lot of time to grow (a single adventure) I would put him closer to the bottom here but still above Terra. All and all though they are ALL powerful mages.
Light and Darkness I think needed their own category because they are forces that heavily affect how powerful our characters are and typically have their most powerful attacks located in them. Straight up in terms of light power Ventus is the strongest for obvious reasons and the only person whose shown a level of light mastery and control that is higher are Eraqus and Roxas. Aqua and Sora are more or less equal in this regard and after much comparison I honestly can't say who is more powerful with light between the two. They both do impressive stuff but part of me WANTS to say Aqua if only because she trained under Eraqus and can do the cool light chain thing. In terms of darkness Ventus is the absolute bottom for obvious reasons and Aqua is tied with him as she never ever uses the power of darkness due to her upbringing. Sora is firmly middle ground he does use the power of darkness here and there and his power of darkness is pretty strong when he uses it but it's not really in his subset of skills. Terra and Riku have the most raw darkness power but Terra's power was strong is uncontrolled, as said multiple times during the game that Terra doesn't have an aptitude for darkness. He's got the power but he lacks the control partly because of Eraqus but partly because he just....doesn't have it. Which is okay not everyone can be a master of everything they do. Riku on the other hand pretty much the supreme master of darkness not even Xehanort's control of darkness can outmatch Riku's which is ultimately why he had to give up on him. Riku also isn't lacking in light power but much like Sora it's a skill set that isn't quite on the top of his best tools.
Acrobatic skills straight up Sora and Riku are at the top. Even in games created by Osaka Sora and Riku always are more mobile and able to perform more impressive acrobatic feats than TAV are. Perhaps it's because Sora and Riku didn't have the skills necessary to get through various hurdles any other way other than by the use of their bodies. A case example TAV don't need to learn to jump up walls when they can create vehicles that let them fly over everything.
This isn't even getting into strengths you can't really quantify. Like again how Sora has a ridiculous amount of childish creativity and is a master of "monkey see monkey do, monkey does your move better than you". Terra's incredibly powerful will that exceeds any strength of darkness, light, or flesh. Ven being split in half so he's forever missing a part of his power and the amazing strength he demonstrated in the few moments he linked with Vanitas gaining access to his full capabilities. Terra's lack of emotional control and Aqua/Riku's basically now perfect emotional control. Or all the various tools that unique to each one like Sora's magic clothes, TAV's magic charms, SR's dream eater companions, the summon gems/charms, etc.... that can further tilt the battle.
All and all I'd rank it something like this..
Sora (Who with the right circumstances can be the strongest or beat people stronger than him)
But all of them are fairly close