Rankings wise it's tough to beat Bitdefender and Kaspersky. Maybe Eset. A lot of the free ones work well but usually with some limitations so that's a tough argument for the casual user. In that case, I flip a coin. Avast, Avira, AVG, etc. Most of the free ones are there to scare you into buying free by using heuristics. My common sense works better.
The best is Dr. Web in my opinion as it offers, alongside a very good AV, a very good firewall for free, suitable for every user as is fast, good and a bit heavy but very good at protecting us. I never had any problems with it so far. I noticed also a very good detection of malware in various tests so give it a try if you want. Thanks
don't forget webroot secure anywhere, for years my best companion with the smallest inpact on performance I know, this compared with Norton, AVG, Eset ande... even F-Secure which is my second best.