I'm going to leave out characters from chi, because I don't know enough about them.
I'll take into consideration, inherent weaknesses some characters have (for example, Terra-Xehanort's weakness of having Terra constantly fighting him from the inside.)
#1 - Terra
I say Terra because we've seen him go toe-to-toe with Aqua without losing, we saw him take on Eraqus one-on-one and win, we saw him take on Master Xehanort and win, we saw him (as the Lingering Will) take on Terra-Xehanort and win, and we saw him hold out for a very long time (as the Lingering Will) against Sora, and even then when he was "defeated" he wasn't really.
(Note: Some may say Xehanort planned on losing, but even he noted several times that his body was getting old and tired and he wasn't in his prime anymore - That's why he wanted Terra as a vessel. His subsequent loss as Terra-Xehanort against Terra, while not seeming to suffer any internal struggle, is another show of Terra's power dominance over Master Xehanort.)
#2 - Aqua
I put Aqua below Terra because we never saw her fight Eraqus, and the only time she fought Xehanort was when he was possessing Terra, and he was weakened by Terra's influence from within, and she still had a tough time fighting him, though she did take on a powerful pureblood Heartless, fought Ventus-Vanitas with the X-Blade, beat Dragon Maleficent... so she has several big wins.
#3 - Terra-Xehanort
Terra-Xehanort was able to go one-on-one with Aqua while fighting Terra on the inside, and almost win. He'd end up with the #2 spot if he wasn't hindered by an internal struggle.
#4 - Master Xehanort
Xehanort as he appears for most of Birth By Sleep, as an old man. He was able to hold out for a while against Terra, and even with his old body he was able to do fast, powerful chain combos and powerful darkness magic. Another big thing we saw him do as an old man was literally destroy the Land of Departure with little more than raising his keyblade to the sky.
#5 - Master Eraqus
Eraqus had to be pretty decent to hold out against Terra for a while, even though he lost. I put him below old man Xehanort because he was defeated by a then-younger (younger than during the game) Xehanort without even being able to land a hit on him. He trained Aqua, Terra, and Ventus as well, so he definitely knows a lot.
#6 - King Mickey
Mickey is supposed to be really powerful, but the only thing we've actually seen him do of any power significance was hold out against Ventus-Vanitas in a tag-team while an apprentice, and take out a bunch of Darksides. That was an instance, however, in which he took like 3 or 4 of them out at the same time, with one attack. So he's got some skills, but while I think he'd be able to hold out against the top 5, I don't think he'd be able to beat them.
#7 - Ventus-Vanitas
Even under an internal struggle, Ventus-Vanitas was able to take on both Aqua and Apprentice Mickey at the same time, and hold out for a while.
#8 - Riku
Yes, I place Riku above Sora. We saw him get beat down by Sora in Kingdom Hearts 1, but that was when he was inexperienced and only using the Darkness. Now, he's got a balance between Light and Darkness, which gives him more potential than a primarily Light-leaning Sora. He's been able to beat Roxas, Ansem (Seeker of Darkness), Xion, Xemnas (in a tag-team admittedly), Young Master Xehanort, and the Armored Ventus Nightmare.
#9 - Sora
Sora's the main character, but he hasn't shown the huge power wins that people like Terra and Aqua have. He did, however, beat Ansem (Seeker of Darkness), Xemnas (in a tag team and separately by himself), Roxas, and Dragon Maleficent.
#10 - Ventus
Ventus hasn't actually had that many big wins, though they've been significant. He beat Vanitas twice, once when Vanitas had the X-Blade, but that's still pretty significant.
#11 - Vanitas
Vanitas had skills, that's for sure. We saw him put them to the test as Ventus-Vanitas, so he definitely has those good skills, but by himself without Ventus' power, he's not as powerful as even Ventus, as we saw when he was beaten.
#12 - Roxas
Roxas held out against Riku for a while, beat Xion, beat Axel, and held out against Sora for a while. Those are semi-decent feats, but nowhere near power wins.
#13 - Lea/Axel
My opinion of Lea as a keyblade wielder is based off of his skills as Axel. His biggest feat was clearing out a huge amount of Nobodies in an instant, even though it did cost him his life, and holding out for a while against Roxas.
#14 - Young Xehanort
He held out for a while against Riku, but lost. I consider his fight in Birth By Sleep to be one where he has a temporary boost in power. He'd just come through time, and it's obvious with how fast he is then in comparison to how fast he is in Dream Drop Distance that the time travel had some affect on his power.
#15 - Xion
She held out against Roxas for a bit. That's about it.
#16 - Kairi
Her usage of a keyblade in KH2 was pathetic. I get she was untrained at killing things, but so were Sora and Riku. Yeah, they were fighting each other constantly, but it was play fighting, they never tried to hurt each other or kill each other.
Honorary Mentions
#1 - No Heart / Prime Xehanort
I don't count him in the main list because he's never shown up canonically. But even so, his fight shows what Xehanort in his prime would've been like (though watered down a bit), and that's absolutely insane. Xehanort in his prime is probably one of the most powerful keyblade wielders that's ever lived, at all.
#2 - Yen Sid
We know he was a keyblade master. And even though we've never seen him fight, Eraqus holds him in high regard, so he had to have been really good to be both held in high regard by Eraqus AND be a Master.
#3 - Genie
Come on, he's Genie. COME ONNNNN
(I realized something, and I want to note that, of the fifteen ranked that fought other keyblade wielders, Terra, Aqua, and Mickey are the only three we've seen that've never legitimately lost a fight. Terra didn't lose an actual fight against Xehanort when he had his body snatched, Mickey never actually fought Vanitas when he was knocked out by him, and Aqua was never ambushed successfully nor beaten in a fight. Do with that note what you will.)