For "free" AV, there's no such thing as "best." There's only "best this week." New threats show up frequently, and cooperation among AV providers generally allows any and all AV providers to keep up to date reasonably quickly. Occasionally an individual provider makes some changes that speed up scans and/or rearranges installation or messaging to make theirs a little easier to use.

In the US, most people get their operating system preinstalled with a new machine, and nearly all builders include an "extended free trial" of either Norton or McAfee. I haven't checked recently, but in the past you could download either Norton or McAfee (basic AV) for a 30 day free trial. When supplied with a computer, the "trial period" is typically either 60 or 90 days.

A very large percentage of people don't subscribe and pay for continued use of either of those, and probably a majority of those who let their Norton or McAfee lapse don't bother to get anything to replace them. This latter group is responsible for keeping a number of malware forms present and prolific so that everyone is continually re-exposed to them. Some very damaging malware forms, easily removed by any AV program, remain dangerous and widespread several years after they might have been eradicated because of the "non-users" (technical name: IDIOTS).

Among the several very good "free AV" programs, AVG has consistently been rated at least among the top half dozen for at least the past 3 or 4 years, making them a very good choice and possibly the most frequently recommended by knowledgeable reviewers. There are several other free programs that are quite good, but with less established reputations for keeping up with new threats; but few who make it into the reviews have slipped back out due to failure to maintain their quality.

Although some of the free programs have begun adding rudimentary firewall functions, I haven't seen any that are more than "minimal," although I haven't looked closely at what's being offered recently.

In addition to adding minimalist firewall functions, some have added cookie blockers, anti-spyware, and in a very few cases minimalist "anti-phishing" features. When looking for a new free AV, it's worth looking at these other things; but the absolutely essential need is for Anti-virus (AV) protection that's easy to keep updated on at least a weekly basis.

John