Too easy: Ash was only a real character in Kanto, the Orange Islands, and sometimes in Johto. But Kanto was the only time he was ever really remotely interesting, and also very likable and endearing too. It's the only time I can buy that he's supposed to be a 10-year old kid, he actually grew and developed over time, and there were actually more facets to his character than just being a "mature" but dim-witted trainer who loves pokemon, has terrible people skills, and is obsessed with battles and competiton. So Kanto easily wins this one for me.
By contrast. he was dull for most of Johto, even duller in Hoenn, duller still in Battle Frontier, at his absolute dullest in Sinnoh, and in Unova he's just a bland, unendearing idiot whose only redeeming quality is that he gets smacked around so much that it's like the writers don't even want us to take him seriously, and that always gives me a laugh or smile.