Mm, I'd probably say Johto. It seems like the most urban of the regions to me though I'm not entirely sure why. XD; It's also had the most interesting atmosphere of all the regions for me. Very little surfing (yay) and not too many annoying caves or mountains. That and it was something new and exciting yet still familiar, which has it top Kanto for me.
Even the big cities in Hoenn were pretty dinky, imo, so I didn't find it impressive at all. :( Definitely bottom of the barrel.