I guess I'm in the minority here, but I prefer the latter Willows. Don't get me wrong- I love her for ever and always in all forms.
I think the Willow I like seeing best is Dark Willow because she's so complex. But I don't think I'd like her to be there all the time and the way I define my favorites is by if I can deal with it over and over and still love it (it's how I rate movies, books, colors...). So there, I'd have to go with season 4/5 Willow.
I like Willow here because she's confident and sexy and just coming into her own. She is totally adorable as nerdy, high school Willow, but in these seasons, she's just coming to terms with her homosexuality. She goes through devastating stuff (like Oz leaving) and is totally heartbreaking, but she still maintains adorableness and hilarity. She is still smart and still quircky and still nerdy, but she has an extra mmf that is not only... wonderful, but complex as well. I feel like Willow is one of the deeper characters on the show and that doesn't really present itself in the early seasons. As the show develops, so does Willow and we gain more and more insights into her character, but they're so subtle, it's not an issue so much as an occurrence. I like depthy, smart, confident Willow. Except I hate that she chose to be with Kennedy in season 7. That's just wrong.