I don't know about "best" but a good place to start would be the Asus Qualified Vendor List. That is a list of RAM that has been tested and known to work with a given board. It's by no means exhaustive nor should you necessarily limit yourself specifically to that list, but it's as good a place to start as any.
8Gb of DDR3 1600Mhz or higher RAM would do you just fine. GSkill, Corsair, Crucial, Kingston HyperX, even ADATA is decent for a low cost. There really is only 3 RAM chip manufacturers anyway. Just make sure you get something with a CAS latency of 9 or less, 1.5V, and something without alot of bad reviews.
It's pointless to spend much extra on RAM as you'll never be bottlenecked by it. I agree with above:
- 1600MHz (faster is generally a waste of money)
- 8GB (ideal for gamers, more is a waste unless you have a specific Video/Photo editing need requiring massive RAM or other RAMDISK/VM need.)
*Two important points:
1) when you overclock the CPU, make sure you don't overclock your RAM (the RAM and CPU are tied to the same Base Clock so you may need to change the RAM MULTIPLIER to set the frequency back where it should be unless your motherboard can handle that automatically.)
2) Make sure your RAM HEIGHT (due to the heatsinks) don't interfere with your CPU HEATSINK.