I never really liked Chuck's line: "Ask not... bell tolls... etc. etc. etc." It felt out of place and real forced... I mean really, as though any human being in that situation wouldn't say "Hey guys, look!" or better yet, "Shit! Shit!!" I guess his line makes more sense in the trailer than it does in the episode.
I voted for Lee's line "It's Over!" and Andy St. John's line had a very close second, mostly because I think the dialogue exchange between them at the end of Episode 2 was brilliant... particularly when Lee leaves Andy to die and Andy just moans "Lee, Lee..." it had chilling effect on me.
Actually, I think Episode 2 had most of the best voice acting, and was the most cinematic of all the episodes. For instance when Lee tells everyone at the dinner table about Mark, the nervous chatter that follows is excellent.
Honorable Mention: Kenny when Lee talks him down to stop the train. His voice acting made me very sad. I couldn't be mad at him again after that.