Avast, AVG and Avira (and to a lesser extent, AOL) are the main popular free resident AV's. Those first three are all good. (Some folk say the AOL AV is too, but I'm not one of 'em.)
Some users have had problems with the auto update on AVG, mainly that update has frequently failed because the server is too busy, or not ready, or similar. But I believe in the end it always eventually updates.
If one examines the results at
www.av-comparatives.org/ over a period of time, they always all score quite well, and sometimes swap places in the detection rate, with Avira usually being slightly ahead of the other two. For free applications, they are generally up there close to the best of the paid ones, and for the average user, that is more than adequate, IMO.
I'm a happy Avast user; it has given me zero problems and helped protect me well, so I don't feel the need to change. I'm sure there are thousands of people would say the same of AVG and Avira.