Oh, Lordy! Wes, Cordy and Xander had the best character development in the Buffyverse IMO. Considering when they started out there was nothing especially special about them: Xander didn't have anything to really offer the group for a really long time; Cordy was mostly an alternate reality of what Buffy could have been i.e the popular girl; and Wesley's main use in BtVS was to stand in contrast with Giles. I am all for Character Development.