For me: Hoenn definitavely. It's simply the most varied and interesting of the lot. It was made to show off all the features of the new Pokemon Advance engine, and it shows. Lots of nice weather effects, the huge aquatic area with the underwater parts, lots of tile variation, the works. I also like the layout, and the fact that there are a number of island cities. (The lack of islands in general is something I really don't like about Sinnoh. It's just one big lump of landmass.)