Gotta go with Portland on this one. I have rode in many a city, and in the US nothing tops Portland. Only Amsterdam comes close.
Portland still has a long-way to go to become a bike mecca. However, it has motivation to keep improving on it's situation. Many of it's freeways have bike paths next to the road (usually it's own path not just a sholder). The downtown area is overrun with cyclist, so the drivers are used to working with us. There are many bike paths and arteries so you can get about anyplace.
I just wish they would re-pave downtown. My rims are having a hard enough time holding my fat-ass up, let alone dealing with all of thr ruts, and pot-holes.
~n