Id say the vibox 4.4GHz quad core gaming pc, but i would probably go for an xps from dell. They seem very well priced and powerful. I would just build my own.
Probably a no brainer, but avoid alienware. They are all about looks then specs really.
I would go with the VIBOX POWER-FX NEW AMD QUAD CORE CPU NVIDIA GT520 1TB 8GB RAM GAMING COMPUTER PC, just because of the graphics card, which is a must have in a gaming computer. However I would suggest a different computer all together.