Marluxia. Mainly because of the loose end that was left -- why the hell did he want to overthrow Xemnas anyway? Am I the only one that thinks there must've been a reason? And if he was against Xemnas, and was existent in KH2, he -may- have actually been an ally rather than an enemy.
Seconds in line would be Zexion and Demyx, because I don't feel they deserved what they got.
I can't help but liken the Organization somewhat to the vampires like what was on Buffy The Vampire Slayer. They have a physical heart, but it doesn't beat or function. Their blood sits still inside their blood vessels unused -- though yes, when they are cut, they bleed. Their bodies are held together and unphased by decay by the mystical energy of the vampire demon inside. When a vampire is staked, they turn to dust; similar to the way Nobodies fade away into bits and pieces of darkness, incidentally.
That being said, I believe something similar is true for the Organization. As Yen Sid said, when someone with a strong will and heart succumbs to the darkness, the empty shell left behind continues to act with said will. The will is apparently strong enough to keep the body from decaying, and still able to go through the motions of being alive (which may or may not include a beating heart, I haven't thought that far yet ^^;), much like how they 'pretend' to have emotions. They may not necessarily require doing these things, like eating, going to the bathroom, etc., but they do them anyway out of habit and memory.
And I completely forgot the rest of what I was going to say. Damn me.
