I thought Wes was one of, if not the, best developed characters of either series. He stays interesting and compelling the entire way, and the range that Alexis Denisoff shows is pretty outstanding.
I always found Wes a great part of the show, whether that was on Buffy or Angel. He's pure comedy relief at the beginning whose only real purpose is to serve as a vehicle to drive Giles' arc that season and to make Giles look cool (which he really succeeded at). After Doyle died on Angel I thought they were going to have a hell of a time replacing him, but Wes was fantastic (still love Doyle though...) in the dorky role before maturing a bit. When the series really hits its stride, Wes is right at the heart of it. His story with Angel, Holtz and Connor was just amazing, and all the later stuff between him and Fred was superb, even though he took on a much, much darker role.
Even with all the changes he underwent, Wes' chemistry with literally every other cast member was excellent. How many other actors or characters can you say that about? Whether he was the grinning goof ball the Council sent to piss everyone off or the dark, haunted shell he'd become in late S5, he always had an impact. One of my all time favorite characters.