I've never watched Firefly, but I was possibly one of the biggest fans of Buffy the Vampire Slayer and Angel.
Buffy: My friends really got me addicted to watching Buffy, and to be perfectly honest I watched the show for every other reason outside of seeing Buffy, herself. Sarah Michelle Gellar was okay and played the role I suppose as good as it could've been. (especially a lot better than Swanson, if you've ever seen the movie)
However I became a huge fan of Angel from Buffy. I followed the Angel character completely and he became my favorite character. From Angel, it went to Willow, who I truly started to love from Season 4 on, as her witch powers grew and grew.
Xander and Anya were my favorite couple, merely because of how they worked together, and Spike was a better fit for Buffy than Angel was, merely because Angel deserved his own dark spin-off. Spike was the humor type Vamp.. whereas Angel was the "alone" type, that didn't need a gang that "teenie-bopper-ish."
I think the greatest mid-series character that came along for Buffy, would've been Faith. If anything, I truly believe she, herself, could've taken on the role of a Main show player and went out with her own spin-off. (Tru Calling, anyone.
) And I think the worst enemy would've been "The First." Which ironically was their last enemy.
I never liked "The First" because it never had a true form. It wasn't one thing, it was "every" thing. It was all "evil" rolled into one, yet its odd how Angel still dealt with evil in L.A.. so apparently the "First" evil.. was only for Sunnydale, not even remotely outside of that town.
Angel: Again, since I was a huge fan of Angel through Buffy, Angel became my favorite show of the two. I believe this show helped its supporting cast to become better than what they were.
While in Buffy, Wesley was nothing more than a stuck-up watcher, who couldn't handle putting a fire out, let alone the situations he should've been capable of taking care of. When he jumped to Angel, even from his debut on the show, he was suddenly a bad ass. (ish) He then transformed into someone who had guts. Someone who used his head to work out problems, and if anything became greater on Angel, than Giles on Buffy.
Meanwhile, you have Cordy. On Buffy she was seen as nothing more than the stuck-up snobbish rich girl, who at the end of High School lost everything. On Angel she transformed into an adult, growing up for the most part from her snobbish ways. And she took on a huge role as the "eyes" to the "powers that be." If anything, Cordy was my favorite female role, even over Buffy, because Cordy played her part to perfection.. without a doubt.
From there you have the entrance of Charles Gun, a Vampire Hunter who loses his sister, then joins Angel but never fully (to me) trusts him. He sided with him, trusted him in the line of battle, but because of who Gun was, I really don't think he ever could trust Angel. You also had the entrance of Fred, who more or less was Angel's Willow, without the witch-like abilities. If anything, I guess she'd be like their Giles since she worked everything out with her head and couldn't really fight a lot.
Moving on. I'd say the greatest enemy of Angel's would be, obviously Wolfram & Hart. If for nothing more because they were the lead enemy in practically every season, even when they took over the L.A. Branch in the final season. Lindsey would've been my favorite character to represent Wolfram & Hart though. Even to this day, I wish they wouldn't of ended his character like they did.
The worst enemy Angel ever had, was ironically himself. In looking back, the final 3 episodes in which Angel and company went to Leore's home to rescue Cordy.. where Angel turned into the "inner vampire" was stupid to me. That whole series of shows could've really been a mid-season type of thing, more than a season finale.
Also, Angel's rivalry with Holtz and the events that lead Connor, Angel's son, to ultimate be raised by Holtz and hate Angel. I loved that so much, even though it also pissed me off merely because I wanted to beat the shit outta Connor myself and just say over and over..
"Thats your real Father you punk bastard!" The character development for Darla was a great favorite of mine as well.
Final Thoughts: I could spend forever talking about either series, and wishing they'd still be around. I think both series ended stupidly. Both ended with a really huge fight scene that ultimately involved nothing of great importance.
Buffy and a ton of extra slayers defeated an army of random goblin-vamp like creatures, no major enemy.. whereas Angel and company defeated a round table of elite enemies, then ended their show in an alley, with random figures coming after them, including a fucking dragon. Talk about stupid.
