I think there is a greeeeeaaaat difference between season 1 and the last one.

He was evil and soul-less and now he's not that's a pretty big change!
But I think tat Willow ties with Spike. She changed SO much from shy little nerdy girl to self-confidant witchy woman!

They all evolve in their own ways, agreement on the Willow one too, but Spike goes from a completely evil, masochistic, selfish git with no soul, to earning a soul and sacrificing his life for the greater good. That's kind of a leap.

from season 1-4 i would defo have to say willow and then most probably faith

agree with abbi and em. he and willow changed the most, but his change was the most magnificent.

I would say Cordelia but thats in Angel, Spike has changed the most i think, Willow still kinda her dorky self :]

It was a tie between Faith and Willow for me. Sure Angel and Spike both evolved also to becoming amazing characters but I think Willow and Faith had more change in their characters.

Obviously!

She goes from good to evil to good again. Spike only does half of that in the actual story.