I can has bandwagon.
Kanto: Lorelei
Lorelei's been my favorite E4 member for a long time (until maybe Steven, since we're counting champions in the E4, I guess). She's not bad (physically), intelligent, and quite capable in her own right on the field. Besides, Water-types = <3.
Johto: Koga
lol ninja powers. Seriously, though, Poison. I can safely say he gave me more trouble than the rest of the E4 just because of his penchant for screwing people over with Toxic and Spikes.
Hoenn: Steven
As I've said before, Steven's one of my favorite League members. Good looking enough to have a nice-sized fanbase (Oh, fangirls), decent with strategy (Oh, Skarmory's Spikes), and Pokemon that can kick tail (Oh, Metagross).
Sinnoh: Cynthia
Who wouldn't want that team?
Out of all four, I'm split between Steven and Lorelei. I'd probably go with Lorelei out of all of them, if only because of my fondness for Water-types.