If I had to pick I'd say CyberPowerPC or iBuyPower but to be honest very few people who use their PCs for gaming would use any of these. Almost all of them build their own (which is relatively easy) or have someone build it for them. Unless you want a laptop, in which case the only one that really makes gaming capable laptops is Alienware. However using a laptop for gaming is generally a bad idea anyway.

NewEgg

A way shorter more concise version of what I said

You missed a contender that would smoke the rest of them. Falcon-Northwest. A lot more expensive. But a lot better PC, and a lot better tech support. That said, Newegg/build your own beats all of them hands down for price/convenience.