I love Firefly but I think Angel was the best thing Whedon ever did, season 2 of Angel was incredibly good, It was like Angel's very own existential crisis. Angel cuts himself off from everybody starts questioning he's motives and then just gives up. There this one scene 3/4 of the way through the season, where Angel gets into an elevator destined for hell, fully prepared to end he's life. He gets this big lecture about the nature of evil and how it can't be defeated. Then the elevator stops the doors open and he steps out into hell, only to find himself back on the streets of LA, it was amazing like something straight out of Sartre, then to cap it off Angel shattered by this revelation, gives up everything entirely goes and fucks Darla in an attmept to loose he's soul.
Some of the most amazing TV. Unfortunately the whole tv series went straight down the toilet from that point on...
As for Firefly, I think the best thing about it is that the characterisations are very real, you know Mal's not a good man but you're rooting for him anyway. He's not a hero or a villian, he sort of lives in that grey area, very human sort of character, with very real flaws.
My favorite Firefly quote has to be the whole exchange with Badger in the pilot. Especially:
BADGER:
What were you in that big war you failed to win, a Sergeant? Sergeant Malcolm Reynolds, Balls and Bayonets Brigade, big tough veteran, now you got yourself a ship and you're a captain! Only I think you're still a Sergeant, see. Still a soldier, man of honour in a den of thieves. Well it's my gorramn den and I don't like the way you look down on me. I'm above you...