My favorite region would have to be Hoenn mainly for personal reasons. Pokemon Sapphire was my childhood. It was the first game I had ever gotten and it is the only one that has deep memories attached to it. For months, I would complete the game and then restart it all over, beat the game again, and then restart. I couldn't stop playing and at one completetion when I was older, I filled up my entire Pokedex (or at least what you could complete in Sapphire), caught all the legendaries, got my Swampert to level 100 including my Kyogre, and could completely destroy the Pokemon League with just my Kyogre fighting alone in my party. I would love to play it again, but life caught me playing and punished me, so I don't usually have time to complete the game again. But, if I could one day, play the game again, I would.