WOW!! Tough decision, but all things considered I would have to go with Inocence. Its very hard to choose, though. Largely because I do consider them one episode. Its really the only 2-parter that has 2 different names, but that's because things change so drastically from on episode to the next.
Now, Surprise was a wonderful episode, oz finding out whats really going on in Sunny-D, and not being shocked at all. And what he says to Willow after she asks if he wants to make out with her (Freeze Frame), the beautiful B/A moments. and of course Xander and Cordy, and its when you find out about Jenny's role in all this. Surprise has a lot of significance, but Innocence wrapped the whole thing up.
Innocence was when we really started to get an idea about what Angel was like pre-soul, and we didn't even really get the whole effect until Passion, but that's a different thread. Angelus is so evil, so mean, and the way he didn't want to kill her, that he wanted to hurt her, REALLY hurt her. He blamed her because she made Angel feel like a man, and Angelus hates that. It was also really heartbreaking the way Giles reacted to Jenny after he found out who she really was.
Surprise had a lot of emotion in it, but so did Innocence, and no one seems to have mentioned that yet. There was a lot of emotion with Xander/Cordy/Willow, not to mention that Buffy's heart was broken, more than broken. And the way that Giles reacted to Buffy at the end was very touching, and Joyce too. And you can still tell that Buffy is haveing such a hard time, she is struggling so much with it, she knows she has to kill him, but she can't bring herself to do it, because he still has the face of the man she loves. Both episodes are very touching, but I have to say Innocence makes for the better ep. Like Joss said- "Buffy sad, show good, Buffy happy, show not as good."
And we all know that Angelus is just cool, and ohhhhh those leather pants, "Hello Salty Goodness,"