AVG and Avast seem to be the two most popular freeware AVs around.
The highly regarded online Virus Bulletin conducts independent tests of AV products. More info and results
here
Edit: Posted wrong link.
In summary the results are:
AVG: 6 passes/20 fails.
Avast: 10 passes/19 fails.
NOD32: 29 passes/3 fails.
I agree with FatDog, except I also find the user interface for NOD32 very intuitive.
My security suite comprises a hardware firewall with SPI, plus all ports stealthed by NAT/DMZ, NOD32, Ewido, PrevX, Adaware, Spybot S & D, Spywareblaster and an online audit subscription to
Security Space
Call me paranoid but I've got a lot of valuable data to protect and I haven't been caught out yet!!