2's my fave, followed by 3. 2 is just a fucking crucible of pain for Angel, and the torching of Darla/Dru just spectacularly caps off one of the most fucked-up relationships in TV writing. The arc is perfectly paced and hits all the right notes, balancing humor with some really dark material that's less comicbook and more human. The journey to Pylea bit at the end is just good-natured fluff that balances out all of the time the audience has spent in the emotional wringer. Plus, DARLA: they took a piece of castoff Buffy Season 1 detritus and turned her into a real three-dimensional villain. Also, the Lindsey/Lilah duality at W&H gets good play.
4's may be my least fave so far. It had too much of that angsty fop Connor, preggo Cordy, and the poorly-plotted debacle that was the Beast/Jasmine arc. The high points -- Wesley's crazy-ass relationship with Lilah, Faith's redemption, and the "zombies inna high-rise" episode -- were fantastic, but the Connor development was so redundant and predictable it became overbearing. And really, let's face it, the Beast/Jasmine arc wias so utterly perforated with plotholes it almost didn't seem like a Whedon series -- were latter series X-Files writers penning the scripts when Joss wasn't looking? Season 4 feels like a Buffy season: more comicbook drama and two-dimensional theatrics than depth.
3's great, but they didn't run with Holtz like I'd hoped. Still, the damnation of Wesley and his rebirth as "hardcore Wesley" is really memorable. Plus BEST SEASON CLIFFHANGER EVER.
Haven't seen 5 yet. Can't wait, either; it sounds great.
1's fun, but it's a little too Buffy. Doyle sucks -- he's a non-character. Wesley had far more potential. The Faith eps, rock, though. Eliza Dushku's such a solid actress AND convincing as an action hero, to boot (quite unlike Sarah Michelle Gellar).