I've tried several but I kept hearing raves by geeks about eset's NOD32 program. Took it for a test drive and liked some of what I saw, even though the install didn't go as easy as it should have.
Ended up buying Eset's security suite and I've been using it for over a year now. I have to say, it's one of the least intrusive of all the programs I have ever used. It does an excellent job and doesn't seem to take over the computer to get the job done.
I'd have to say it's the best program of it's kind I've ever used.
eset.com. They have free trials if you're interested. Do some research on it and see how it rates.