what is the best motherboard for mini ITX?

Rammy

Honorable
You can't really do AM3+ at anything below ATX.

ITX boards basically don't exist at all (though there have been a few exceptions in the past).
mATX boards are hugely limited in selection, to 760G chipsets which are fairly antiquated.

If you want smaller motherboard AMD builds, you are stuck with FM2/FM2+ boards, though depending on your price range and usage demands, you might well be better off looking at the Intel alternatives, as they are much better supported across the range.
 

Rammy

Honorable
There's not exactly a huge selection. - http://pcpartpicker.com/parts/motherboard/#s=23,26&xcx=0&sort=a8&f=8&qq=1
If you want to cover yourself for future upgrades, you'll need one of the two FM2+ boards.
If you don't, then it's just a case of matching up any features you need.

As a side note, if you were intending on using an FX4300, then it stands to reason you are using a discrete graphics card. If this is the case, then using an APU (A10/A8 etc..) isn't really good value as you are paying partly for internal graphics you won't use. An Athlon X4 750K or 760K is similar to the A10s but with the internal graphics disabled, and significantly cheaper.