Hoenn. By far.
Hoenn is my favorite region until now. It was a little bit tropic, wich I loved, but also the variety. It has some big city's (Rustburo, Mauville), some small towns (Littleroot, Oldale, Verdanturf) some exotic town's (Lavaridge!), some islands (Dewford, Mossdeep), and my personal favorite, some water city's (Pacifildoc, Sootopolis).
Also, there were some forest's, an ice cave, and a meteor cave.
There were also some pokemon wich I liked the most of all pokemon regions. Most of my favorite poké's came from Hoenn.
There was introduced some new thing to do: Contests!
Also some small things wich I enjoyed like the two evo's from Wurmple, a cool team (IMO), Secret Base's, and more.
It also contained some things wich were worse then in R/B/Y or G/S/C.
There were the worst trio in this game; the Regi's.
I thought it was some little bit too hard to discover them, and, I think that a full defensive trio was some little bit to much.
I like defensive poké's, but not when it's one full trio of defensive poké's.
And I thought that Groudon, Kyogre and Rayquaza were better then the other box mascots.