The best depends on how it is to be used and the budget. The 8-cores are the "best" however the one mentioned is a 95W CPU which may be below the FX-8320. THIS one from the support page:
http://www.asus.com/Motherboards/M5A78LM_LX/#support
FX-8300(FD8300WMW8KHK,3.2GHz,8C,95W,rev.C0,AM3+)
I'm not certain which CPU this is. It can't be the FX-8350 or FX-8320 as they are listed as 125W CPU's.
You don't discuss a BUDGET however. None of the AMD CPU's are as good as a modern Intel CPU. The i5-4440 in particular is a good deal, just in case you haven't yet bought the AMD board and are open to other options.
The FX-6300 is (arguably) the best value for the board you mention.
**WARNING**
Not all CPU's for this board will work without the BIOS being flashed. See the link at the top. It's possible a newer version would have the most recent BIOS but you need to find out what BIOS version it has to see what CPU's are supported.
SUMMARY:
So my advice is the FX-6300
http://pcpartpicker.com/part/amd-cpu-fd6300wmhkbox
Verify the model and BIOS but I believe it's compatible.