There's stuff like Perfect World that can simulate Earth in DF, but this is for the specific REAL real world, with all its geology and nature and little idiosyncracies intact. The only thing that's missing is modern-day humans. Hell, let's make it more interesting: Worldgen was 200 years ago, and there's quite a few Savage places lurking around. Good and Evil is wherever you want it to be. Oh yeah...and no FPS restrictions.
You could pick a nice mountainous spot in the Rockies, but do beware of the skeletal eagles and the goblins that might lurk in the foothills...and of course the South American jungles are full of delicious trees, but the Giant Jaguars would tear you to shreds, and then the elves would find you. Hawaii might be a good choice for its volcanos, but does it have enough valuable ore...and more importantly, would you get bored with no goblins? (Surely there's HFS, at least.) Antarctica is always good for laughs, especially if you like +penguin tallow roasts [20]+.
Me, I think my first instinct would be Yellowstone. It's probably Untamed Wilds, but there's volcanic activity and rivers.
Bonus points for looking up the ores that are present, and for linking the coordinates on Google Earth! If you've got a specific mountainside you want to dig into, all the better.