The best that I've used (in terms of lowest memory footprint, intrusion etc) is NOD32. It's not free, but I'm more than happy to pay for it.
For free you're restricted to AVG, Avast or Comodo. Avast seems to be the most popular, but frankly, they'll all vary from month to month over which is most efficient. Obviously they're not as good as the (real) proper products, namely Kaspersky and NOD32. Apparently the free AOL one (OHNOES!!) is based by Kaspersky, so could be quite good if it's not been filled up with AOL bloat.
Norton is absolutely shocking in every conceivable way. It currently has quite a good detection rate, but frankly the way it kills virii is by slowing the PC down to such a crawl that the virii die before doing what they're supposed to. McAfee is better, but still in the same boat. Their enterprise scanners are much better mind, Symantec's (Norton) inparticular.
Edited 1 time(s). Last edit at 04/10/2007 02:20PM by gav.