Man, there's so many, you'll just about have to check 'em out yourself to find what you like. I work in downtown Denver, but live in Parker. I've driven through lots of what look like nice areas near downtown, but I don't "know" anything about them. What some might consider good neighborhoods might not suit your tastes.

I'd suggest that you come for a short visit to explore around, if possible, use a rental car and motel if you have to. Find out where you're gonna spend most of your time (school, work, etc.), and start looking in that area.