In short, ESET, Kaspersky, Symantec, McAfee, and Microsoft come out on top when they were tested by third parties. Of those, ESET and Microsoft were the only ones on top in most every test, while Kaspersky, Symantec, and McAfee had some less than top-notch test results. However, Kaspersky and Symantec typically top most tests recently.
Of that list, Microsoft is the only one with a free product called Morro. But they currently sell OneCare/Live (not free) and Morro is in Beta, though it will be free soon. I wouldn't recommend relying on a Beta version of any anti-virus product since it defeats the purpose. Only use Beta if you want to help the company test it, not for actual protection. The third-party tests for Microsoft evaluated OneCare/Live, not Morro. Morro is supposed to replace OneCare/Live and Defender.
Of that list, ESET and McAfee are the least expensive (per individual purchase, not by licenses).
Of that list, ESET and Symantec are the only two with a long-running, proven track record of doing well in third-party tests. Others (except for McAffee) have decent track records, but haven't been around as long as these two.
If you want the "best", pick from that list, somewhere along that line.
If you want something 100% free that is still rated as acceptable by third parties, use Grisoft's Free AVG, Alwil's Avast! Home Edition, or Avira AntiVir Personal.
^--- ALL OF THAT LISTED ABOVE IS FACT, NOT OPINION ---^
Now my opinion: you get what you pay for. AVG and Avast are probably going to be a better option than Avira, since Avira has good detection but overkill with false positives (which is extremely annoying). But, I wouldn't use a free anti-virus when I can buy better and less bloated commercial anti-virus software for $30-40 a year.
And I wouldn't want to use Symantec because it's bloated. I wouldn't use McAfee, because their products fluctuate in the tests (one month they're great, the next they don't cut it). I didn't list others like Panda, F-Secure, etc., because they don't come out on top of others in third-party tests. In fact, some of them fail on many tests.
I personally use ESET because of its performance (thought not the best in every test), its small footprint (not bloated), its good detection of viruses in the wild, and its ability to run in a "silent mode" that stops all pop-ups, update notifications, etc., except when it actually detects a virus or malware.
The terms "viruses" and "malware" are sometimes confused, interchanged, and their detection can vary in different anti-virus package. Malware is used as a global term to cover anything bad software that is not identified, and can sometimes be used to cover viruses, though it typically covers things like spyware, adware, and other unwanted software. Many anti-virus applications will run well in conjuction with anti-malware software such as Microsoft Defender (which is free). Some anti-virus software will protect well against malware and not require separate anti-malware protection.
Also, you won't want to use multiple anti-virus applications on one system since many of them will clash with each other. Some products will even warn you when installing them on systems with existing anti-virus proteciton.
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