I have to say Gen III for many reasons.
The first reason is the Pokemon. It had the coolest starters, cause you can't beat a kickass fighting chicken, a chubby axolotl and the king of the forests.
Then there's Lilycove City as a whole. The beautiful music, the department store, contest hall, the painting place. It's my favourite location in the entire series. On the topic of cities, there's just so many different ones, the basic towns, the robust Rustboro City, Sootopolis (which is situated in a former volcano), two towns on the side of Mt Chimney, a little jungle village where everyone lives in trees.
The variety of so many places, Pokemon, things to do in Generation III as opposed to the prior generations, where all the towns and cities were kind of similar with the only real things to do in the games being battle, catch and gamble.
So yeah, Gen III sits at the peak of my personal Pokemon mountain.