I'd say Rogue Demon Hunter. He was great on Buffy, but mostly only for a little laugh here or there, or to cause tension for our heroes.
But on Angel, not only did they ramp up the laughs, (Seeing Wesley fumble and trip over himself, all the while flaunting his cool leather jackets and weapons and "Rogue" demon hunter-iness always made me smile) but he got a true character ARC! We see him go from the same, self important, prideful, pathetic watcher wanna-be, to a true, full on hero. And of course, we also get some tormented Batman eras as well...
He was great on Buffy, but he truly shined on Angel.