You should then define first what are the mesurement requirements to be "the best"?
For me, personally, I define it like this: very high detection rate of the live-shields, low system impact, inexpensive, good overall coverage.
My conclusion is this (again, only my personal rating): avast has a high detection rate without many FP, it uses very few system resources, it is free and it has all the features that need to be paid in other solutions (e.g. mail scanning and web shield compared to avira).
This makes it "the best" foe me.
Of course one may come to other conclusion. If you want 100% detection, you will maybe use avira and take a higher FP-rate.
I just think that avast is the best overall-package, well balanced between efficency and impact (both on my computer and my purse)
And again: only my opinion and personal rating.