You've left out the guy from GTA Advanced (I think his name was Mike maybe?) and the guy from Chinatown Wars (whose name I don't remember even though I got 100% in that game). And why not add Jimmy Hopkins from Bully to the list.
None of them stand out to me as particularly better than the rest. I want to say maybe Luis is a better character than the rest, but maybe that's just because he's from the most recent GTA that I've played.
I'm torn between Niko and CJ.
Niko because, despite the whole sociopath mass murderer thing, he seems like a guy I'd actually hang out with. He's weirdly affable.
CJ because I really like his character arc. San Andreas had the best sense of escalation in the series (even though it became wacky and cartoonish as it went along... not that that's a bad thing).
Tommy or CJ. How does Niko get so many votes? That asshole got annoying because i just couldn't relate to his ignorance of being in America. And how about the fact that Claude blasted Maria at the end of GTAIII (awesome) but this dude couldn't shoot Roman to shut the fuck up and stop fucking up everything for them. Or why he did any missions for Vlad? I just didn't like him.