I haven't had the chance to ride my fixed there yet, but Fargo, ND would be near the top of my list.

1. There are no hills. Whatsoever. The only elevation gain is when a road goes over a highway.

2. The winters are nasty. That really lets a fixed gear show it's strength.

3. Thriving downtown area with wide-ass sidewalks. I'd prefer to ride in the road, but with sidewalks so wide and deserted, why not ride on them?

4. No sophisticated bicycle theives, and a (relatively) small town. My sister's bike got stolen there (she left it locked up with a cable at a bar overnight). She found it two weeks later, locked up with a chain. My dad cut the chain and they stole her bike back. Fargo rocks.