I really can't decide. Willow is just too pretty! It's honestly like a tie between S2, S3, S4 and S6. She also had her good moment in S1, S5 and S7 but her hair was just too long in S1 (although I liked the auburness of it) and her outfits were too dowdy. I didn't like the S5 curly hair and some of her outfits were matronly although I loved her short straight hair in the beginning of the season. And while she's so very pretty in S7 and I love her hair, she's more dour and colorless than I like My Willow!
So I'm pretty undecided between S2, S3, S4 and S6. I think she looks equally great in short and long and shoulder length hair. I loved a bunch of her fashion forward outfits in S4 and S6 but some were just too outlandish. I loved a bunch of her more simple and unassuming outfits in the high school years but some were just too childish. It's nice seeing her more made up in the later seasons but she carries off a fresh-faced look very well in a bunch of the high school eps.
I guess I'll compromise with a tie between S2 and S4 for Later and Early Willow.