Johto, probably the only game where I wanted to use all Pokémon.
Typhlosion was one of the few strong Fire Pokémon in G/S/C - Flame Wheel is one of my favourite moves- , Meganium is a strong Grass type and Feraligatr is an awesome water type. I think Totodile was the first one I chose. Then it was Cyndaquil. The Johto starters are special to me for Silver was my first game. ^^
Otherwise: Hoenn
Grovyle's Leaf Blade is another one of my favourite moves, along with Blaziken's Blaze Kick. I don't care what you say about Blaziken, he wàs good. =O Plus the combination of Fire and Fighting was original. I never train Fighting Pokemon, so it was nice to have one for a change. ^^
Same goes for Swampert, but for a Ground Pokémon.
Kanto and Sinnoh are my least favourites. Charizard and Blastoise are awesome Pokémon, but I'm not really a fan of Venusaur. And Sinnoh, I guess I only like Turtwig. ^^"