lol
Well, Saix is definitely evil, and that, I assure you, is not a word to be taken lightly, but I say that it is Xemnas. Xemnas is spawned from a madman, is power-hungry and sees and monitors those ignorant enough to think they're fooling him, around him. And, not to mention, the fear he inspired on Vexen in CoM was quite remarkable. I mean, he is practically Xehanort, only thrice as insane and powerful. AND not quite as preachy, which, for a villain, could be a good or bad thing.
Saix on the other hand is mad, yes, but not nearly as ingenius or powerful. And, in all honesty, if he really was as evil, then he would be the main villain, because he's the only one that comes so close. Which, he really does.
But Xemnas seems to be using him too; he let him get so close to Kingdom Hearts and did nothing; it was like he knew Saix was sure to be killed. Let's face it, as amazingly cool as Saix is, and as cool, he's not quite as malevolent as Xemnas was. And the others really don't compare to these psychos.