I think all four of the main male characters have some form of AS. Trust me, I've met (and am friends with) plenty of Aspies who don't exactly SEEM Aspie (I wish I could count myself as one, but people generally seem to detect that there's just something "different" about me, lol), which I why I think this. Sheldon, of course, as many of you have already said, is the ultimate Aspie in the show!!
I consider myself to be more of a Leonard personality with Sheldon-esque quirks
Leonard certainly has a better handle on social skills and less particularities than Sheldon, though I still wouldn't disqualify him from being Aspie - IMO, he has SOME traits of it. Raj already has the "confirmed" diagnosis of selective mutism on the show, but it seems like other times (though not that often) he can just be more generally clueless, so I'd also count him as a member of the spectrum. And, yes, I've EVEN known some Aspies who are like Howard - both the sexual aspects AND the overconfident aspects.
As minor characters go, Leslie is the second most obvious Aspie to Sheldon IMO, and Barry, in spite of the way he talks, seems to alternate between being Aspie and just plain old jerk (though he is more so the latter). Bernadette merely LOOKS "nerdy", but probably isn't an Aspie.
And Penny...well...she's like that stereotypical "normal" girl that a "nerd" and/or Aspie might be likely to have a crush on, esp. in high school (I know the characters are older than that, but Penny seems more like a high school stereotype than the other characters on the show).