My personal favorite "Rachel" episode is the two part "The One That Could Have Been" (season 6, episodes 16 & 17). It showed how she might have been had she married Barry. My favorite line by her in that episode was, "I wish that we could be on a break or something." when referring to having an affair with Joey. As far as best season, I don't really have one because every character has good and bad episodes every season.
Edited on 11/20/2012 2:15pm
Edited 2 total times.