I agree with you all, it's hard to chose because she's great in all seasons for diferent reasons. It's really the characters that embodies the best the passage from teenagehood to adulthood, & the characters it's the easier to identify with (for me).
She is so cute in the beginning & I agree, she makes you want to hug her, so innocent & naive & all... & then gradually, it's like she becomes conscious of her feminity & therefore she grows up being more a "woman" than a girl.
But if I had to choose, & even though I really like a lot Willow S6 because she is the prettiest there, I would still chose the Pink-Pullover Willow or the Willow in overall of the ealier season, cause it's just awwww.