What is the best quad core for asus p5q deluxe?

I can't say what is best

but I am using C2Quad Q9400 Intel 2.66 GHz 6MB cache, with P5QL-PRO for quite some times

and yeah it is working fine

As long as it's on the CPU list for that motherboard, any Intel socket 775 Quad core will work fine. If you plan on using Windows XP mode in Windows 7 (or Hyper-V in Windows Server 2008), make sure that the CPU has support for Intel VT.