Leon's campaign: The whole thing with Deborah, partly due to the great work of Helena's voice actress.

Although the death scene could have been made more touching with the right music instead of (almost) none at all.
Chris's campaign: The conflict cutscene after Marco's death. Damn, I'll never get tired of watching Chris and Piers argue, and every time I see it I'm waiting for the anger-filled, passionate kiss, but it never happens!
Jake's campaign: The motorbike sequence. Although it wasn't so much fun playing it with my friends and dying
18 times because they were just as good as Leon at driving a vehicle...
Ada's campaign: That's hard... The campaign offers many great moments, story- as well as gameplay-wise, but I think my favourites would be her conversation with her "dead" doppelgänger and the fight after that. It's so beautifully creepy!