Golly gosh, such a hard question that should be easy. It is all about the interactions between ownership and the mechanics. I spoke with a local owner last week, and he seemed to think that mechanics are commodities that are interchangeable. I happen to think not, but the fact of the matter is that the job does not pay all that well and some owners think they can have a young person with little bedside manner doing the wrenching, since it is just a commodity.
Bicycle Space downtown is no more, and that is where I ALWAYS took my bikes. I like the owner and the mechanics were crusty and honest. They are now just in Ivy City and I don't really know who is left.
The Bike Rack is downtown and seems to have great mechanics, but I have a palpable dislike for the owner, so I don't go there. They seem to win awards for best bike shop each year, so worth a try.
When it comes to servicing my older bikes, the Bicycle Pro Shop always has parts in stock and very good mechanics who were nice to me. I also had some success with the Conte's in Ballston, depending on who did the wrenching.