I've tried a lot of antiviruses: Avast, AVG, Avira from the frees, Kaspersky, Panda and NOD32 from the paids.
My favourite from the frees has to be avast. Easy to use, not a memory hog, catches a lot of viruses.
But my favourite of all antiviruses is NOD32 (actually Eset Smart Security, which is the whole package). Yet to find an infection it won't remove - even those fake antiviruses.