Kanto he was the funniest and interesting to see a rookie.
In Hoenn Ash actually seemed, "cool." Its the only saga where Ash actually seemed like he was a big-shot.
In Sinnoh he was definitely the strongest he ever was, but I felt his personality started becoming too bland. Its a shame really, I missed the humor side of him.
I think BW Ash is the worst because he's been regressed for no reason. Johto Ash was also fairly dull, like a bizarre mix of his Kanto self and what he'd eventually become in AG.
Losing ONLY one Gym battle in a completely foreign environment is NOT regression.