Well, being as how I started with Kanto, I will have to say Kanto. Oddly enough, I prefer all the middles. Ivysaur, Charmeleon and Wartortle have always been some of my favorites.
Other than that I do like Typhlosion and the entire Totodile line. Chikorita line never did interest me much, so I haven't used it a whole lot over the years.
I do like all final forms of the Hoenn starters, but not too fond of the other forms.
Never played any Gen IV-V so I can't comment any farther.