No such thing as best anti virus.
Some have good protection but slowing down the system, others lite, but will not protect you as good. Best Anti virus is carful user. Any way here are some recommendations. To make it simple i will not give to many options here.
Kaspersky - Really good protection, but resource hungry, not free and the Internet security version bit complicated for non experienced user. If you have good computer and you want the best protection, Kaspersky is my favorite. (the only one i consider to buy at the moment).
Microsoft Security Essentials -I was surprised my MSE. It is free, provide dissent protection, really easy user interface, easy on your resources. Best free solution i think.
The only downside i can think of, MSE will automatically enable Microsoft update. Not every one will like that (for different reasons).
Avira - Free, super lite, good protection. Avira is grate, but not perfect. The free version will give you pop-up window ones a day, some false positive,( if you programmer or you have custom applications, be careful). Best description is "shut first ask questions later" I think Avira is the best free solution for slow home computers.
Looks like TheTron gave you a good rundown of the top anti-virus software. Choosing one is sort of a personal preference. I'm using Microsoft Security Essentials on my Windows 7 desktop at home and haven't had any problems. The best part about trying out the free services out there, is that if one is not right for you, you can move on to the next. If you'd like to learn more about Microsoft Security Essentials visit http://windows.microsoft.com/en-US/windows7/what-is-mic...
Hope this helps,
Microsoft Windows Outreach