Best: Anything with Janice
Although one of my all time favourite pieces of dialogue was in the The One with Ross's New Girlfriend:
Chandler : Yo, paisan. Can I talk to you for a sec? Your tailor... is a very bad man!
Joey : Frankie? What are you talking about?
[Ross enters and touches Chandler on the shoulder, who flinches]
Ross : Hey, what's going on?
Chandler : Joey's tailor... took advantage of me.
Ross : What?
Joey : No way. I've been going to the guy for 12 years.
Chandler : Oh, come on. He said he was going to do my inseam, and then he ran his hand up my leg, and then there was definite... cupping.
Joey : That's how they do pants! First they go up one side, they move it over, then they go up the other side, they move it back, and then they do the rear.
Joey : [Chandler and Ross stare at him] ... What? Ross, would you tell him? Isn't that how a tailor measure pants?
Ross : Yes. Yes, it is... In prison! What's the matter with you?