Snow. God that gets boring so damn fast. White everywhere you look and it reminds you of being cold as ****. Hail in every damn random encounter where you fight ice pokemon, overall a horrible type to catch.
Desert and snow, because of the eternal sand-/snowstorms. Caves are OK, as long as you pack enough Repels.
Although the caves were a lot worse before Gen V, because of the "puzzels" they had you solve. I'm talking about the use of TM's/HM's like Flash and Strength. Not to mention the maze that was Victory Road. I don't mind having to explore a bit, but the layout of Victory Road was deliberately obtuse and therefore not very enjoyable.