My experience is not extensive, but:

1. Denver-Boulder is fabulous for bicycle commuting 9 months a year. The area has very decent infrastructure for bicycles, and even when you share streets with cars in vehicular cycling mode, the local culture is generally pretty tolerant. They do have winter, though, which means you need a plan B once in a while...

2. Seattle has mediocre bicycle infrastructure, but they're working on it. More importantly, there are very large numbers of cyclists here, the drivers are generally used to them, and almost everyone has learned to deal with a road environment where not everyone is in a car.

3. San Francisco is pretty much like Seattle, only the hills are steeper, it's more crowded, and bicycle parking is more limited. (Not as many bike racks as Seattle.)

4. Portland is nice, but, hype aside, isn't much better than most west coast cities. Outside of the central part of the city, it's actually a lot more hostile for bicycles than either Denver or Seattle.

5. LA: Not the best place for cyclists, but a whole lot better than you'd think. It might actually be better than most places in the US.

6. Berlin: It's not Copenhagen or Amsterdam by any stretch, but it still kicks a$$ on any place I've used a bicycle in the US. Its bicycle infrastructure is uneven and obviously pasted together haphazardly, but it's in place, everywhere, heavily used, and supported by traffic laws that support bicycling. It's also totally flat, which means you can ride in reasonably formal attire and not ever have to worry about sweating too much.