I really do like of them equally, but Sinnoh gets extra love, because it's where I started.
It has a great sense of history, and Dialga gets love from me, cos I like anything that can manipulate time. Must be why I love Homura so much :P
Hoenn has a lovely atmosphere about it, being all water and stuff. I love coastal locations, so Hoenn was a treat for me.
Unova, the industralised and modernised region of the Pokemon series. All sleek and fresh. It's hard not to get lost in Castelia~
Johto! It's all diverse, in my opinion. I haven't really played enough of both Gold/Crystal and SoulSilver (the Johto games I've played) to get a feel of the land, and I don't really see much of it anymore, but it seemed like a really peaceful region.
Kanto and Kalos I haven't seen enough of, sadly...