Rachel - ExFx
She's the hardest to type, even though she's probably the main character along with Ross. She doesn't fit as an ESFP, neither as an ENFP. Maybe ESFJ, but I can't find the "judging" (J) part on her, just can't.
Ross - ISFJ
I used to think Ross is a "T", but I've read one comment in this thread and I changed my mind. He's for sure an Introvert. I don't see him as an iNtuitor, as many here typed him, since he's more practical and down to earth, seeing things as they really are. He's indeed a feeler, not a Thinker. And the "J" is the most obvious; Ross is a work-oriented, organized person.
Chandler - INTP
The borderline between his E and his I is so thin, it's hard to define, but I'd go with an "I", because puting aside his frequent jokes with everyone, doesn't remain lot to make him an "E". I can't really see a iNtuition preference on him, but he doesn't strikes me with Sensation at all. The T/F reason is pretty unclear too, but I'd go with Thinking.
Monica - ESTJ
ESTJ does fit her very well. She's a down-to-earth extrovert who doesn't show many feeling traces. Her Judging preference is mainly clear when she shows her inclination for clean and organized things.
Joey - ESFP
There's no doubt about it. Some people here typed him as an "T", and I can't see why. He cares about other people too much, and he shows that. I remember the episode where he were trying to say sorry to Ross, because he "accidently" asked Rachel in marriage, this makes his "F" preference very clear to me, and other facts too...
He likes to have fun, is silly and everyone likes him. Even though this describes ENFP too, ESFP is perfect for Joey.
Phoebe - ENFP
I wouldn't discard the INFP option, but she is clearly a NFP. NF temperament with perceiving: totally Phoebe.