Frankly, the free versions just don't measure up to the paid for versions. Some are better than others, but how ell they perform chnages in almost every test. The paid for versions can oft be had for as little as $10 a seat.
In Decembers test, Avast Free AntiVirus 2016 did best among the freebies
Protection against 0-day malware attacks, inclusive of web and e-mail threats - (140 samples used ) = 98.3%
Detection of widespread and prevalent malware discovered in the last 4 weeks - (14,658 samples used = 99.8%)