I ran internal testing at my old company to choose a corporate anti-virus package that was flexible, easy to use for our end users and cost effective.
After all the testing, it came down to 2.
Eset NOD32
Kaspersky A/V.
Nod32 won hands down in terms of speed, but its interface was clunky/not user friendly. Heuristics were even with Kaspersky but Kaspersky had the advantage of an easy to understand UI for end users so I went with that.
I highly recommend the above 2 A/Vs. I recommend that you actually purchase an A/V as most of the free ones aren't 100% dependable when it comes to certain threats.
If you are going to use a free anti-virus and are unable to afford a pay for one, then use Avira. Internal testing ranked it pretty good against KAV & NOD32.