Topher, fully realised Echo, Adelle DeWitt and Mr. Dominic- in pretty much that order.
To me Topher is an interesting 'funny' character done right. He's funnier than Xander and way more fascinating imo. (and also, oddly more likeable dispite his massive flaws)
Fully realised Echo is (and could've been) a fascinating character to watch develop over time, add to that she's an unstoppable bad-ass action hero most of the time. I wish we'ed seen more of her because then she'd be my top- almost certainly.
Adelle is just pure, calculated, morally certain, dryly British and unflappable. Capable of being deeply caring and brutally practical in the same episode. Brilliant.
I just love the fact the Mr. Dominic is an unapologetic asshole.

He's like 'yeah, so what? Deal with it.'
Least favourite character: Paul Ballard by far, he's kind of a massive cliche (and not an inverted one either- rare in a Joss show) without that many layers of interest. Although he does get a little more interesting as the show progresses.
Plus his pursuit/romance of Caroline/Echo is mostly just creepy.
