Tests and results vary. All I know is that Avast catches at least one virus in my incoming mail most days. I don't know what I did to upset the virus-writers, but they seem to want to send their offerings to me with alarming frequency. Avast has never failed me. AVG did well before Avast, until the day it locked up my PC and had to be surgically removed!
I do pay for the "basic" Avast - I'm not quite a freeloader!
I'm getting annoyed at the false positive "Spam!" reports, though. I get several of those a day, all on genuine stuff from bank or credit card etc companies. I turned spam reporting off, but of course Avast complains about that.