Good question...
As I was thinking about each body part, I realized that I don't really have a least favorite to work, but I do have least favorite exercises! Lunges are evil. I do them without weights so I make sure I have really perfect form. They still hurt without the added weight, by the way.
Sometimes I really hate plie squats, although I don't mind the regular squat. Push-ups aren't thrilling, although I do the bent-knee version. I LOVE deadlifts, don't like upright rows, love abs, hate leg-lifts....
oh--but I really love things like ice cream and chocolate, (and you know which body parts they gravitate to...):9