Hoenn. Even though my best memories are from Diamond (and Platinum), Hoenn just stands out as the most beautiful region. With its epic towns and nature, dayum, you just gotta love it.
But Sinnoh, I love that too. It's pretty cool especially in Platinum. Whoa.
Then I guess, Johto. Kanto and Unova share the last spot, since for some reason I don't find them that cool.
(everything has to be cool)