For the free ones (for personal use), Avast is pretty good. I didn't like it on a laptop since laptops aren't on all the time, and thus the pattern file updates wouldn't be continuous (and when Avast gets behind, it seems to hammer the hard disk for a short time after boot trying to hurry and scan everything to catch up).
I really don't like Symantec -- it's a resource pig.
If you're willing to pay some *****, I REALLY like eSet's NOD*2 -- doesn't seem to have any performance effect on the computer at all.