Wow, this is really tough. I'm torn between James, Amy, and Alyson.
James was just spot-on in every scene he was in. He could do a scene that was funny, sad, moving, or anything in between and never seem unconvincing. And he was able to completely switch personas without skipping a beat, which is crucial when playing a character like Spike who has frequent spurts of anger. He was incredible as evil Spike and good Spike. Also, his British accent was flawless. Who would have ever guessed he was an American?
Amy is just...wow. She was 100% believable earlier in Angel, when she was just a nervous, shy girl who had just moved from Pylea, and I always thought it was amazing how she could ramble through her lines without stumbling once. But she was able to really show what a good actress she was when Fred turned into Illyria. My God, you couldn't even tell it was the same actress! Her personality and voice were completely different from Fred's, and still, she was always convincing. And it was amazing in The Girl In Question, when she was able to shift from Fred to Illyria in an instant. I'm surprised she hasn't been in more stuff since then.
Alyson...okay, I may be just a bit biased, 'cause I'm pretty much one of the world's biggest Willow freaks, but her acting was still undeniably great. Whether she was nerdy Willow, fuzzy Willow, witchy Willow, or evil Willow, her acting was always convincing. Also, even though most other gay/lesbian relationships on TV seem awkward, forced, or both, her relationship with Tara was always completely believable. Plus, there's something about her that just gives you total sympathy for her; when she's sad, so are you. Oh, and, as Joss has mentioned quite a few times, she's a great crier.
Then again, I'd also be happy to vote for Sarah, Tony, Eliza, or Alexis...God, there's so many great Buffyverse actors! I'm not even sure if I should vote. *wishes she could pick multiple people*
By the way, I agree with whoever said they didn't think David was all that great. I mean, he's definitely a good actor, but he's not quite as good as a lot of his Buffy/Angel counterparts. His acting as Angelus seemed very forced, and his Irish accent was 100% cringe-worthy. And sure, he could lead his own show, but that's only because Angel had a great ensemble cast.