For Buffy, I guess the question excludes answers like "what in general did you hate about the show." So I will say the sex at the dumpster in Doublemeat Palace. I already hated Spike, I hated the fact that they had a sexual relationship, and this bad sex humiliation just makes me cringe.
Mal ,'Serenity'
Buffy and Angel 1: BUFFYNANGLE4EVA!!!!!1!
Is it better the second time around? Or the third? Or tenth? This is the place to come when you have a burning desire to talk about an old episode that was just re-run.
For Buffy, sadly, I'd have to say most of S7.
On Angel, one of them was the Rat Pack reference in The House Always Wins, because continuity? Not so much.
Also truly, truly hated Cordelia's descent into St. Cordy, and the Angel/Cordy love (shudder).
Least favorite Buffy: S5 arc getting Riley off the show. Xander's speech being an actual moment of feeling very disconnected and "Are we supposed to think he's right?"
Least favorite Angel: Ascended Cordy looking down from the Heavens in white. Feh. One moment to sum up my entire dissatisfaction with that storyline (though I'm also very sure that story was driven by meta issues).
HSQ Angel: Drusilla vamping Darla. Just the way she glided in, silent, horrible, beautiful.
HSQ Buffy: I was spoiled for so much...Hmmm. I knew the plot moments including Giles comeback, so it was more the lines of dialogue that made me go Whoa. Like when Willow challenged Giles (before going evil) that was a huge turning point for her character.
Least favorite for Angel is hands down St Cordy, followed by the champion thing and this:
Angel - The flashbacks in TGiQ. Way to emasculate the Scourge of Europe.
Buffy, I would have to say PodGiles and the potentials and speechifying in S7.
Ascended Cordy looking down from the Heavens in white.
Wasn't she yelling something? It was all very goofy.
Oh, yeah, and I hated the puppet.
I hated the puppet.
Freak. Maybe if he'd stalked Fred or something, you'd have liked him more?
Oh, yeah, and I hated the puppet.
Wha...?! I loved Puppet!Angel. "My nose comes off." Heh.
For Buffy, my favorite HSQ moment would be "Don't worry, dear heart. I'll see that you get strong again. Like me!" at the end of "What's My Line." For Angel it's a toss up between the reveal of Darla in the box and "And yet somehow, I just can't seem to care."
I am as ita with regard to my least favorites.
The whole "But he's changed now!" school of justification for S7 Spike.
First, for violating show, don't tell.
Second, for basically using the same line that shows up whenever someone takes an abusive partner back and taking it at face value. (I think if you go back into the Spoiler archives, I've ranted about this at length.)
Third, for attempting to have their character cake and eat it too by not making the changes in the character all that major.
Jilli and I have discussed at some length how they could have possibly made it work. Visual cues, like the inch long roots of basement crazy, oddly enough, would have helped a lot.
I'm well aware that I'm like the only person who had a problem with a tonal shift from "dark and disturbing drama" to "show that cites to Fraggle Rock."
Freak. Maybe if he'd stalked Fred or something, you'd have liked him more?
There is no level of creepiness that will get me attracted to muppets.