It's between Hoenn and Sinnoh for me as well for various reasons. Hoenn just happens to be my favorite region in the games and the Pokémon there are pretty exotic, so it's a win for me. Also, May was introduced along with Contests, which I'm a fanatic over and I enjoyed seeing her participate in them more than I did with Dawn. It's also the series I really focused on since I missed out a little bit on Johto. :\
Sinnoh wins me over because of its plot, rivalry, and battles. It by far has had the best plot and rivalry between Ash and Paul; Paul being probably the best rival for Ash since Gary IMO. A majority of the battles were done really well, which is what I enjoyed. Plus, the contests with Dawn had some eye catching appeals. Though, the Ice, Water, and Fire appeals became a little bit too much after awhile.
So, between Hoenn and Sinnoh it's hard to say exactly which region I prefer. Though, because Hoenn happens to be my favorite generation and May being my favorite character, I lean more towards Hoenn than Sinnoh.