It depends on your definition of "best". Best OC, best price, best looks, etc. are all possibilities.
Any brand name DDR3 RAM listed for AMD systems "should" work with the FX CPU. At the same frequency and latencies all different brands of RAM deliver virtually the same results. Obviously the default frequency for RAM with a FX CPU is 1866 MHZ but some folks just run 1333 MHz. or 1600 MHz. to save money as no significant performance is lost.
Independent objective scientific testing by Tom's Hardware, AnandTech, X-Bit Labs and even OCN's "damric" show that in real applications there is no significant increase in system performance when the RAM is operated at ~1333 MHz. Significant for this discussion on RAM means if a person can't visually see or feel the difference in system performance, it's not significant. Synthetic benches will always show a gain with increased frequency or tighter latencies but in real apps people rarely ever see significant gains.
Llano/APUs are the exception as the GPU section can use faster RAM frequency up to a point for some modest gains.
That being said if you want to OC the RAM just for the sake of OC'ing the RAM then any of the name brand RAM rated 1866+ will be fine. Many FX CPUs will run the RAM OC'ed to ~2200 MHz. If HSF clearance is an issue then look for DDR3 RAM without top mounted heatsinks as they are not needed on DDR3 RAM which only uses ~10w of power. Some people keep the RAM frequency close to the default and just OC the CPU clockspeed for the biggest performance gains possible.
I would not recommend paying a premium for tighter latency DDR3 RAM either as one clock cycle in REAL time diminishes as the frequency increases thus there is no tangible system performance gains there either, as objective testing with real apps has confirmed.
Since Asus mobos IME can be RAM sensitive I'd e-mail both the RAM maker and Asus to confirm compatibility of the specific model DDR3 RAM you intend to purchase BEFORE you purchase to be certain there are no known issues on the Asus model mobo. The RAM does not need to be on the QVL list to work properly. In fact most QVL list which are done when a mobo model is first released are typically out of date and never all-inclusive.
You'll likely want to use the latest mobo BIOS also as the mobo makers are still sorting out all of the features of FX CPUs.
In "theory" all DDR3 RAM should work in all mobos but reality has shown this not to be the case for some mobos/RAM.
As with all RAM the SPD settings are to insure the PC will boot so if you buy RAM rated faster than 1333 MHz. you will typically need to go into the BIOS and manually set the frequency and latencies and then save these settings and exit for higher frequency operation.Edited by AMD4ME - 5/18/12 at 5:31pm