Anti-virus software is not a security panacea. Additionally, IMHO there is no “best” anti-virus.
Here are some links to help you find what is best for you.
AV-Comparatives – Real-World Protection Tests
Virus Bulletin – Latest Comparative
PC Mag – The Best Antivirus for 2013
Personally, I would not pay for an AV solution as I think the free versions of these programs are good enough. (I am NOT a security expert.)
If you want the full (pay-for) versions there are often deals that make these programs free after rebate.
Try the usual deal sites, Techbargains, or Newegg.
See also:
Why Antivirus Software Isn’t Enough To Fend Off Attacks