I personally use AVG 7.5 and it seems to be working great in the past I have used Norton (now I like to refer to as System Never Works), PC-cillin, NOD 32 (which is damn good but also very expensive) and, Mcafee.
I would say AVG free is probably your best bang for no buck
I also have been using AVG spyware removal tool and must say it is most excellent.