My one and only complaint for Hoenn would be the removal of the day/night system. I really liked that feature from Generation 2 so I had a hard time letting that go.
But other than that, I loved Hoenn. Especially loved the new species additions (Seviper, ftw!) and the Diving!
The weather was great, too. Falling ash? Yes! I don't know how many times I ran around collecting ash for that glass furniture! Which brings me to my next gush - Secret Bases! I didn't care too much about them at first, but once I started to play around with the concept and started exchanging records with people, I really became hooked.
I can't say that I had as much enthusiasm for Sinnoh... even with the return of my precious day/night system. Sinnoh just didn't really pique my interest as much. I just didn't feel as motivated to explore and discover as I did with Hoenn.
And as someone mentioned earlier, the music wasn't all that inspiring either. The only theme I can think of when I think Sinnoh is Route 209's theme... and I only remember it through Super Smash Bros. Brawl!
Meanwhile, I can distinctly remember several times in Hoenn in which I just stood around listening to music. Especially in the Oceanic Museum and Sootopolis.
To summarize: "Heck Yeah Hoenn!"
(I guess we'll be seeing a R/S/E remake soon at the rate things are going, eh?)