Worst member? Wow, I don't know if there is one in my opinion.
Xemnas - The leader and has a pretty sick look for a badass and is fun to fight. The Laserdome was seriously fun.
Xigbar - His voice and his expressions are so funny, and I love how he fights upside down with a gun. I love those comments during your fight with him: "R for Reload", "So whaddya think", "Yeah, now we're talking", "I got a dance for ya", and "Now, let's see how you dance." I also love his "As if" catchphrase, not to mention he's a main part of The Organization who knows more than he's letting on.
Xaldin - I personally think weilding all those spears and controlling the power of wind is wicked awesome.
Vexen - I'd say he's useless (Wow, I found one after all), because of that scream he makes whenever you hit him. I do, however like his Ice Shield, and how he can take Data from you to summon an Anti-Sora.
Leaxaeus - If not for his awesome weapon and attacks I saw in Kingdom Hearts 2 Final Mix 2, I'd say he's garbage.
Zexion - He's cool because he's an enigma. Not to mention he's big with the Emo Crowd. He fights using Illusions and Incantations, or whatever is in that book to fight which is pretty cool. There's more to him than what people already know which I believe deepens the thought of knowing who he really is. I like peopl like Zexion. It makes you wonder more about him.
Saix - Not only is Xemnas' right hand man, an important main character, a HUGE part of the story and The Organization, but he fights like a Madman with a whatever his weapon his called. I like thosed combos he pulled off. I personally like him. His X-Shaped scar adds to his coolness factor.
Axel - He uses fire, he's practically best friends with Roxas, weilds those Chakrams, and he's a key factor in KH2.
Demyx - I don't think he's useless. He's kind of the comedic act of The Organization. He did get serious during his fight though. Fighting with Water and a Sitar is pretty cool.
Luxord - If not for that perfect Movie Voice and the way he turn fights into a game, I'd say he's the bottom of the Totem Pole.
Marluxia - I say he's important. It's a bit fraky about his care for Roses and all, but that's what makes him what he is, Besides, he tried to take over The Organization. On top of all that, he uses a Scythe.
Larxene - I like her because just because she's the only female member (well there is the 14th), but she makes her prescence known when she has to. She owns people with Thunder. I've never seen Thunder used like that.
Roxas - Do I need to say it? Dual Weilding Phenomenon, Sora's Nobody, Twilight Hero, and is one of, if not, the main story of Kingdom Hearts 2. I loved to use him and loved Dual-Weidling against Axel. I saw a video of the Final Mix fight with Sora, and he pulled off moves I've never seen before.