The two that absolutely, positively took me by complete and utter surprise:

Buffy - "I'd like to test that theory." Could practically hear the chorus of "GILES!!!!" sound far and wide.

Angel - Dru vamping Darla. Just so cruelly perfect that twist was.

And since I came to Buffy late, I didn't get the benefit of a lot of HSQ moments being truly unexpected. Still, Giles made me so happy that time. Awwww.

This question is too hard.

Buffy: A toss-up between Angelus snapping Jenny's neck, and Angel dropping in from hell, naked. I want to say it's Giles's "I'd like to test that theory," but I knew he was coming back. I squeed, but I wasn't surprised.

Angel: The pillow is it, although locking the lawyers in with Darla and Dru is a close second.

This question is too hard.

Hee. I thought maybe making it "favorite" instead of biggest or most important would help. Guess not.

I came into Buffy late and internet fandom even later. But, once I did I become one of the most thouroughly spoiled on the current seasons (6 and 7, for me.) Fortunately, I didn't find the internet sites until after I'd watched numerous reruns on FX. In fact, my first ever viewing day provided a heck of an HSQ moment for me. I was watching a Thanksgiving Day marathon and since they skipped a lot of episodes, my biggest HSQ was watching Buffy vs. Dracula and at the very end Dawn shows up. Since I didn't have the benefit of the entire earlier seasons (only the select episodes shown up to that point) to know of all the foreshadowing and was totally unspoiled, both my daughter and I turned to each other and said, "WTF? Buffy doesn't have a sister?!?"

By the time I started watching Angel (season 4), I had read so much of the earlier stuff and was so spoiled there was really very little that made me sit up and take notice. So, the only HSQ moment I can honestly remember was in season 5 and that was during the opening to Power Play where Angel bites Drogan. I never expected that, I didn't think Angel had changed so much he would do the expediant thing rather than the right thing. I was sad, angry and so disappointed at that moment, I almost didn't want to watch that or the final episode. I felt like we were getting regurgitated season 2 and it stung. I realized some of that was because they had to wrap the season up so quickly, but still. It was HSQ on a couple of levels for me, actual doing and narrative-wise.

Yeah, 'WTF SISTER?!?!" is my favorite HSQ moment in Buffy, too. It's just so... weird. Sadly, "Two to Go" was my first Buffy episode ever, and I had no clue what was going on at all, so "GILES!" did nothing for me.

In Angel, nothing ever went as far as Doyle's early death for me, though a really large portion of Season 2 came very close.

Yeah, 'WTF SISTER?!?!" is my favorite HSQ moment in Buffy, too. It's just so... weird.

Ack. I totally forgot that one. Maybe because I'm not a huge Dawn fan. But I was unspoiled for that, and I was like, "OMGWTF?!"

I will say again that Wesley's introduction on Angel made me about fall off my couch, which just goes to show I should sit further back.

However, it wasn't the same sort of !!!... !!! moment as the pillow.

HSQ, in general, was less effective for me on BtVS. I think most of the things that would have seriously made me yelp happened in S2, when I was a much more casual viewer and not invested enough to care.

I want to say it's Giles's "I'd like to test that theory," but I knew he was coming back. I squeed, but I wasn't surprised.

see, THIS is why I avoid spoilers like the plague. there's nothing like the HSQ for something like that. I was watching with maybe 10 people, and we all SHRIEKED "GILES!" there were high fives and whatnot!! it wouldn't have been the same if I'd known. Plus, when I'm spoiled for something (by accident or because I don't care), I just wait for the spoliery bit to happen, then enjoy the show.

I wasn't even spoiled for Giles's return, I just *knew* that he'd be coming back for the final episode. So while I still get a little racy thrill from hearing that line and seeing him in the doorway, there wasn't as much holy-shit! as there was there's-my-man!, you know?

Angel: pillow; lawyerfest; Justine in the closet

Buffy: naked Angel; Giles killing Ben/Glory; dead Joyce