I have been using BitDefender, due to a lot of good reviews and so far so good. It doesn't seem to bog anything down and only really warns of things that I care about (registry changes). It seems to be a very good value.

I have used Kraspersky and AVG. I like Kraspersky better, but all are suffecient, imo.

And because you can't say it enough: "Norton SUCKS!"