If you're talking about generally, Hoenn since it's been ages. However I'm more looking forward to rebattling them on the potential remakes so the World Tournament might spoil it a bit for me. But World Tournament wise, Kanto could be interesting.
I would love to rebattle the Unova gym leaders, after defeating the main story line, before battling the champion (especially Lenora - I would love to to have another Watchog vs Watchog battle [that was fun!]) with their Pokemon much higher levels. Make great practice before the champion.
Sinnoh and Hoenn for sure. I have some lessons to teach Tate and Liza with my Darkrai's and Milotic's...>83 Bwhaha...
And also because I just want to destroy the Sinnoh leaders again because It's fun. xD.