It depends on your needs. As a home user, I use Grisoft AVG with no problems. At work I have been evaluating Eset NOD32 on my laptop and it works great. Our email filter uses Sophos. A few years ago my research brought me to the conclusion that Command Antivirus was one of the best, but since then they becamse Authentium and I don't know how well their AV is now.
Want something free? I've heard good things about Avast (and AVG is free too).