Best very subjective. Never been a fan a the big names because of all the bloat and hogging of resources. I've used Microsoft's own solution since the MS Anti-Spyware beta days right after they bought what is know known as Windows Defender from Giant Software. A few years ago I decided to go with Malwarebytes Anti-Malware Premium for the real time protection in combination with Windows Defender. I know neither are really considered true antivirus solutions, but I find them adequate for my needs. Lightweight non resource hogging security software.
Malwarebytes Anti-Rootkit, which is an application that has been in beta for years, is also excellent. It has a very powerful scan engine. It's an on demand only scanner with no real-time protection though. At work, the stuff that our BitDefender Business Edition can't remove, the stuff Malwarebytes' own corporate application can't find, their beta Anti-Rootkit program finds and removes with no problem. BitDefender also comes back with a lot, and I do mean A LOT of false positives where I work. The Malwarebytes products are extremely fast. I've had BitDefender running on employees computers for 4-6 hours doing a deep system scan and find nothing but cookies. It's awesome when I run a BD scan, and since it looks for cookies first thing, the 'Infected Items' count is 30+ right off the bat and people get all freaked out that 30 'viruses' were found in the first 30 seconds of scanning. I can run MBAR on the same computer and it will complete the scan in 15-25 minutes depending on how many files there are and it will find a half a dozen or more legitimate threats and remove them.