Ive been asked this question and can never just say one but nobody really outshines anyone in Angel in my opinion, I like them all so i'll say my fave characters in Buffy.
Male: Xander, Closely follwed by Oz
Xander because he's been by Buffy and Willows side from day one, has no powers but is incredibly brave and loyal and would give his life for his friends, also in a dark situation he always lightens it with his humour.
Oz because he said so little but what he said was quality, and Willow and him were the cutest couple and seemed to be so much in love, i think they made a far better couple than Willow and Tara and Kennedy
Female: Buffy and Faith
Buffy because, well shes the star of the show and the show could not exsist without her. She has been through so much but no matter what happens she always makes rational and balanced decisions and has coped with so much pressure and diaster.
Faith because she brought a bad slayer - good alyer vibe to season 3. Se showed us what could happened if Buffys decisions were different and just produced an incredible storyline. Shes also very relatable because even though she has superpowers she is vunerable and her own badness spawned from her insecurities and need to fit in like everyone feels at some point. Also, she doesnt take no crap and had a bit of an edge to her.