Okay, there are FIVE main Pokemon regions in the world.
They are [in order]: Kanto, Johto, Hoenn, Sinnoh, and Unova. Sevii Islands doesn't count. Because there are no official cities, league, etc. Same for Orre, Almia, Fiore, etc.
Then you explain why that region is your favorite. Is it because of the Pokemon that reside in that region? Is it because of it's climate. Or because of it's cities and geographical features?
Here's my thought:
For me it is easily my favorite Region. I love the diversity that Hoenn has that most other PokeRegions don't have. It has a more humid environment. It introduced deserts to the games, it brought most of my favorite Pokemon. I love Hoenn's cities. From the urban feel of Rustboro City to the tropical paradise of Dewford to the mystical wonderful city of Sootopolis. Hoenn, really, is a beautiful region.
Simple, hmm? What's YOUR favorite region? :D