A line that gets a HUGE laugh in the episode where Ross and Rachel break up over the 18 page letter:
Rachel: And hey! Just so you know, it's not that common, it doesn't happen to every guy, and it is a big deal!!
Chandler: I KNEW IT!!!
Rachel's line is great, but for me the timing is off as Chandler comes around from behind the door and responds MUCH too quickly. She was jabbing at Ross about something that happened to them in private (I hope!) and there was no time allowed for Chandler to process what she had said and to make any sense out it that fast. It bugs me every time!