Nuno Bettencourt, Yngwe Malmsteen, Eddie Van Halen, Glenn Tipton, Kenneth Downing, Hendrix, Luca Turilli... I can't choose one out of those and some more, I don't think we could just pick one at all.
I don't really know much about guitarist, but I do know Slash, so I'll just say he's the best. Also, I do like Jimi Hendrix because I actually do know him and he was such and influential guitarist.
um.. i'd have to say van halen, he mastered the guitar, and had to write guitar riffs on piano so it was actually challenging.. thats pretty awesome.. why does almost everyone pick someone from guitar hero? just because they're on guitar hero doesn't mean they're the best! and also, everyone picks hendrix.. idk, that kinda makes sense.. lol but still, Van Halen!!!