My favorite cry is easily Victreebel. It actually sounds like its singing, as opposed to the various grunts, growls, and static that permeate the world of Pokemon cries. I've enjoyed Victreebel's cry since the days of R/B/Y.
I've always had a thing for Seaking's, to be honest. :O I like Natu and Xatu's too, and Voltorb and Electrode's, of course. The Regis sound very awesome as well.
Absol's and Furret's. You get fond of your favorite pokemon's cry, espically after you raise three or four to lv. 100, though I know other people may have done more.