Besides the main stars (Buffy/Angel), who had the biggest character developments in both series?
No, wait, wait. Buffy for sure. Seeing her transformation from superficial Valley Girl to the mature, confident, brave and strong woman she is now? She is a truly incredible character. Buffy has sacrificed everything for her mission and has remained strong with the weight of the world on her shoulders.
That's a hard one.
I do think Cordelia should get a mention. She went from a spoiled, self centred cheerleader, to helping people selflessly (especially when she was in pain from those visions), to being part demon, then a higher being then completely evil and finally a confident, self assured woman. If you look at what happened from were she started in Welcome to the Hellmouth to Your Welcome, I'd say her development is pretty similar to Willows.
Similar to Willow's? ...I don't really see it. Willow went from mousy to confident to arrogant to dark to... well, more S4-type Willow but with more power and control. Cordy doesn't count as turning evil because she was possessed by Jasmine. A completely different being took over her body; that's not character development. Everything else besides the 'completely evil' stage is right, though.
I don't know, if you take Cordelia from "Welcome to the Hellmouth" or "The Harvest" and then watch her in Angel S4 or her farewell episode the next season, she's a totally different character. Sure, she still makes a few of the same jokes, but that's true of most of the characters, even Willow. Heck, they even do that teenage rewind episode in S4 of Angel where Cordy reverts back. I'm not saying she has the biggest development, but it's pretty huge.