I voted for Amy Acker. She's a brilliant actress. Fred and Illyria were as different as characters can get, and she played them both without missing a beat. Also, when most people play their characters for the first time, they're not really sure how to act them out, but Amy seemed to know her character from the very beginning. That's the mark of some great acting. I can't wait to see her in Dollhouse!
Honorable mentions go to Alexis, Alyson, and James. Alexis was just brilliant. His transformation from the dorky, stuffy Watcher in the beginning of Buffy to the badass, depressed Wesley at the end of Angel was incredible. He's definitely underrated; probably because he wasn't THE main character, and he hasn't really taken on any super-prominent roles since Angel. Even though Alyson had the occasional episode where her acting was a little bit off, her acting in the other episodes more than made up for it. She WAS Willow, from beginning to end. And, even though we were used to the lovable, dorky, innocent Willow from seasons 1-5, she still managed to creep you out as Evil Willow. But the best part about her acting is her ability to make you feel bad for her. When Willow's sad, you're sad. James? Well, he's got a flawless British accent (as does Alexis, but he lived in England for a while) and his performance as Spike was always spot-on.