I'm not sure I could pick a favourite as I love them all!
I did think the final series had some great bits in it.
I loved Joey on the quiz show, Pyramid, the category is "Things you'd find in a fridge"
Pyramid Partner: It's white.
Joey: Paper... snow... A GHOST?!
Thanksgiving when Rachel makes trifle with beef in it;
Ross: "It tastes like FEET!"
Ross and his teeth whitening
Ross and the tanning booth
Assistant: Alright Mr. Geller! Right this way! So, how dark do you wanna be? We have one, two or three.
Ross: Well... I like how you look, what are you?
Assistant: Puerto Rican.
FANTASTIC!!!!!
Loads more great quotes here:
http://www.imdb.com/title/tt0108778/quotes