I'd say Spike, Willow, Wesley, and Cordy.
In the very beginning, Spike was a lovesick poet. When he was sired, he stayed romantic, but he also became a killing machine. A couple hundred years later he got a chip in his head, and kept on killing, but this time he killed demons. Then he fell in love with a slayer and tried to become someone good to earn her love. He got himself a soul, got his girl to fall in love with him, and saved the world. After that he stopped saving people to impress Buffy, and instead did it because he knew it was the right thing to do. DEFINITELY evolved a lot. 😀
Willow began as an innocent, book- smart, sweet little girl who was hopelessly pining over her best friend, Xander. She doubted herself and didn't think she was anything special (as hinted in Wrecked: "who was I Buffy? Just some girl." ). As shown in Restless, she thought that she was weak and pathetic, and often tried to hide herself(Halloween). Over the next few years she started gaining more confidence and independence, started becoming her own person. She became stronger and started making more mature choices. In S6 she became extremely powerful, and the power almost corrupted her. Then in S7 she learned to harness that power and use it for white magic, which eventually turned her into a goddess. Needless to say, she evolved into a very powerful and beautiful woman, who WASN'T always hiding in the shadow of her Slayer best friend.
Wesley, Wesley, Wesley. He began as a slightly oblivious, bumbling, poncy young bloke. 😀 He was rather awkward, yet still incredibly intelligent. When he crossed over to Angel he matured and became darker, but was still kinda conceited. A couple years later, he was doing what he could for the "greater good", regardless the consequence. He turned nonchalant about killing, and was very tough. He died for his cause, and still to this day it's hard to tell what he believed in, exactly. 😀
Cordy's awesome. 😀 She started off as a spoiled, popular, arrogant "Queen Bee" girl. She only hung out with the "cool" kids, and made fun of the not so cool ones. A year later, her daddy lost all his money, and she was kicked out of her popular group for dating Xander. She befriended the Scoobies, and though was still overly honest, started acting a bit nicer, or at least more accepting. 🙂 When she moved onto Angel she became wiser and sweeter, but still retained that funny honesty thing, which is a good thing, because it makes her genuine. When she came back in You're Welcome she was a nice, heroic, champion, loving, loveable woman, who had matured into a gorgeous, respectable young woman. 😀
Honorable mentions go to Faith, Tara, Dawn, Darla, and Gunn... And also Lindsey and Lilah, they changed a lot. 😀