Sinnoh. The main reason being that i enjoyed the games in sinnoh more than the ones in Hoenn. Now i know it's about the region and how it looks but the enjoyment you have in the games contianing the region I think affects what you think of that region.
I did enjoy the crater village... if it was more flexable and what i mean by that is that there was only one way. You had to surf to get to either pokecenter or mart.
The dive feature was cool... if only it offered more. You only got two pokemon from the weeds on the bottom and a few hidden items. There was that secret place to get the regis unlocked but what a pain in the *** that was to get to. The main advantage i could see from the dive option was the fact there was no random encounters there unless you went in the weeds.
Yeah it was nice to have a jungle village, floating village and one on the slope of a volcano apart from not being able to run on the bridges or on the logs which was a bit irritating. Also the ashy route to the volcano place was interesting. However it contains one of my least favourite pokemon of all time, Spinda.
The pokemon found in each region i think is accountable for the region's enjoyment aswell. All of my least favourite pokemon are from the 3rd gen (Volbeat, Spinda, Nuzleaf, Slakoth) and not because the are third gen but because they have traits i do not like about them. Saying that some of my favourites are found in Hoenn such as Beldum, Absol and Swellow.
Sinnoh however offers the same pokemon and more. Yes it has Bidoof and we all know what everyone thinks of that pokemon but it offers more of my favourites and there is still a nice variety of weather and landscape. And it does have some of my least favourites too however usually they come in swarm mode, therefor i bump into them less. The snow was a nice addition although i'd rather not get stuck in it.
Really all hoenn offers in extra to sinnoh is the underwater place and the strong current route (which is irritating).
The map does have two levels, don't forget the underground which is by far better than the underwater part as you can get alot of items and have fun with your friends. And technically you can get more pokemon there in the form of fossils.