The areas around Whiterun and Falkreath to be sure (when it comes to outdoor areas), but I'm a sucker for exploring every single dungeon. I'm still finding dungeons I've never been through before-it really is amazing how much content they put into vanilla Skyrim. Though with that said, I have to mix the dungeons up. I can't mentally deal with doing 2 or 3 Dwemer ruins in a row. Those falmer just creep me out. Plus the variety is the spice of (virtual) life!