Buffy- S5. Buffy is truly The Slayer here, she deals with real loss as a grown up, has responsibility thrust upon her, takes on a Hellgod, wins, but has to sacrifice herself anyway to save her sister and the world.
Willow- S3. Willow at her most likeable, supportive, sweet, brave and clear-sighted about what she wants to do. She doesn't know yet how much the magics will cost her, or change her.
Xander- S7. No mopeyness, no constant self-doubt about his place in the Scoobies. He's earned his stripes and respects himself after saving the world with his mouth. This continues on into the comics of S8, where he's one of the best characters there also.
Giles- S5. Giles finds his purpose not just as a Watcher, or as an adult, but also as Buffy's unwavering Father figure, Buffy's rock, supporting her in times of hardship. Where this guy goes in S6 or 7, I have no idea.
Anya- S5. I liked her best here, before her sctick wore pretty darn thin in S6, and then her character became a waste of time in S7.
Spike- S2- I'll take evil, chipless Spike the Big Bad. We never see him anymore, not even in flashback.
Dawn- S7. She's mature, fairly wise for a teen, especially in the beginning episodes before the Potentials show up. She does get ignored after this but MT still gives a likeable performance.
Faith- Angel S4/Buffy S7- More on Angel than Buffy but I like mature, reformed Faith best. Reformed and redemptive, sexy and sassy, mature but without losing her kickass edge. Should've had her own series after this.
Angel- Buffy S2/ Angel S4- These two seasons we see the most sides of Angel, petty, heroic, brave, jealous. Plus we have Angelus as well. 🙂 Oh, and 'Orpheus'. Genius.
Cordelia- Angel S1- I know she grows into the 'heart of team Angel', but I like me some shallow, snarky starlet Cordy best of all, plus Cordy/Doyle, plus her constant arguing with Wesley. She grows a lot, even before getting the visions.
Wesley- So many to choose from, but I'll go with S5 of Angel. Because we get all sides of Wes here- bumbling, slightly clueless, awkward, dark and dangerous, drunk and deeply unstable, comic and tragic.
Gunn- Angel S2- When they knew what to do with him and he felt like part of the group, instead of a bystander. In S5 he becomes relevant again, but loses something of his character in the process.
Fred- Angel S4. I don't particularly like her romance with Gunn (its kinda boring) but her smoldering with Dark Wes...and she's pretty much the only thing worth watching for most of the Jasmine arc, when she's all alone and trying to figure stuff out.
Connor- Angel S5. I can't hate Connor because of who he turns into, an amazingly well-adjusted super-powered teen who gets on well with his Dad, and comes to help him save the day. Season 5 Connor is awesome. 🙂