For me, it's AVG all the way.
I have tried the free versions of Antivir, Avast and AVG, and personally I much prefer AVG.
Avast took longest to scan, then Antivir, and AVG takes the least time.
Also, they went in that order from most resource intensive to least.
These are only my personal experiences and opinions remember, but I also liked the interfaces in the same order, from least to best.
So for me, AVG gets full marks, Antivir slightly less, and Avast much less.