In the past 10 years I've had phones on all the carriers at least twice (I'm even including Nextel).

The only carrier I've been pleased with both times I've used them has been T-Mobile. I like their customer service, I LOVE the innovative stuff they're doing now, and their CEO is a lovable crackpot.

In the DC area, all the carriers have nearly identical coverage. There were a couple years where I had TMo and my wife had Verizon, and we matched signal almost everywhere.