There's a couple...
In the one where Phoebe is dating Mr. FreeBalls, Gunther has a great line, possibly my favorite from the whole series. As the guy's junk is hanging out of a pair of gym shorts, "Hey buddy, this is a family establishment. Put the mouse back in the house."
Top episode in my mind is the flashback one where it shows the alternate universe. Where Monica is still fat (and looking to give away her "flower"), Ross is still with his wife, Joey is on Days, Rachael married Barry. It always cracks me up. The follow-up to that epsiode is great too. The "what if?" factor makes it fun.
is that a light at the end of the tunnel or just a train?