James Marsters aka Spike
Marsters hung around Buffy for longer than the rest of them, but eventually popped up on Angel in its final season as Spike once more. Besides that - some guest appearances - Without a Trace, Torchwood and perhaps most memorably as Milton Fine on Smallville. Marsters has also appeared in a few movies - both TV and wide release, such as P.S. I Love You. He will be appearing in the upcoming live-action Dragonball film as Lord Piccolo.
And we can't not mention...
Seth Green (Oz) appeared in a string of short-lived TV series (Greg the Bunny, Four Kings) , as well as the highly successful (and entertaining) film, The Italian Job and will reappear in its 2011 sequel. Also, perhaps most notably, Green voices "Chris Griffin" on Family Guy and is a writer/producer/voice actor in his own series, Robot Chicken.
Eliza Dushku (Faith) appeared in the short-lived supernatural-esque series Tru Calling and will be headlining Joss Whedon's new show Dollhouse, premiering next Friday on Fox. She has also appeared in a number of films included the well received wine film of last year, Bottle Shock.
Michelle Trachtenberg (Dawn) has a knack for played 'hated' characters. She will soon be reprising her role on Gossip Girl as villain Georgina. She also starred in Eurotrip and Ice Princess, appeared for 4 episodes in Six Feet Under and starred in the recent ABC Family original movie, The Circuit.
Emma Caulfield (Anya) has appeared in some TV movies including "I Want to Marry Ryan Banks" and "A Valentine Carol". She did some voice work for a few episodes of Robot Chicken (as have many other Buffy alums); they must all be friends with Seth Green! She has also guest starred in Monk and appears to have 2 movies on the horizon for 2009.
So...tough call, but which Buffy alum do you think has stolen the show in their post-Buffy days?