As my sig quotes often enough indicate, I am a huge fan of Buffy, but a bigger fan of Angel (AtS). Haven't yet seen Firefly, though.
I like Buffy as a coming-of-age series that at times asks insightful questions about the nature of good and evil. Angel, though, is the "deeper" and more adult of the two series to me. The philosophical questions it asks are harder, the Los Angeles scenescape it is projected against is grittier than the suburban hellmouth (literally!) of Buffy, and it delves at least somewhat into white/black race relations. To me, though, the piece de resistance is the final season of Angel, in which Angel and his team (Gunn, Wes, Fred, Lorne, Spike, and to a limited extent, Cordy) are offered - and, seduced by the power, opportunity, facilities, signing bonuses, and opportunity to "change the system from inside," accept almost immediately - the leadership of the Los Angeles office of Wolfram & Hart, the evil law firm against which they have been fighting for five seasons. They immediately find themselves trapped between their desire to wage their traditional fight against evil, and their newfound angry, powerful clients, who want them to advance various evil objectives (and might retaliate themselves if they don't). The questions the final season poses deal with just how blurry the line between good and evil can become, and what good people (and, y'know, good vampires) caught inside a system a bit too big for them to change from the inside can do to escape the maelstrom.
As two characters say in Season 4 of AtS, in an episode named "Habeas Corpses":
Wesley: There is a line, Lilah. Black and white. Good and evil.
Lilah: Funny thing about black and white: you mix it together and you get gray. And it doesn't matter how much white you try and put back in, you're never gonna get anything but gray.
The entire series is about fighting to maintain the separation between black and white, and how to find the good when it is lost in a sea of ambiguous moral gray. Good stuff, especially when discussed through an always-compelling plot. I highly recommend it to everyone.
Plus, you know that any show that can get me to sit through 5 seasons - that's 110 episodes, 50 minutes each - of "Attorneys are evil" - has to be pretty darn good.
The lawyer jokes are endless (and funny).
Cordelia (to Angel, who is a vampire): You were just soulless, blood-sucking demons. They're lawyers!
Angel: She's right. We were amateurs.
(ETA: but the show has nothing to do with *law* per se)