I currently use all three (Avast, AntiVir, AVG) on multiple different computers and here is my very quick review:
AVG:
CONS: Probably not as good detection rates as others. Non-standard and tired-looking interface. Trouble with updates a lot. Just doesn't ever seem to find anything. Maybe that's just because I'm careful though.
PROS: Never bugs you with popups, ads, or notices to buy their other stuff. Light footprint. Seems stable and never crashes. I've used it the longest of all, 4 or 5 years now, on a few systems, never an issue.
Avast:
CONS:Better interface then AVG, but still a bit weird. Uses two icons in tray unless you "merge" them. Has some configuring to do after install.
PROS: Probably best detection rates. Also no popups. And light footprint.
AntiVir:
CONS:Throws popup to buy their stuff usually after each auto-update. It's free, but still have to register, and even still, says it will expire. Not sure why a free program expires, but I guess you just have to remove it and reload a new one?
PROS: Cleanest interface, really like it. Small footprint. Scores pretty good on tests, daily updates.
I can't choose any particular one, I use them all, and so far can't really say one or the other. I would pick AntiVir if it didn't expire, and have a popup. I would pick Avast if they toned down the interface and the loud British guy who bellows out at 3am over my speaker "ANTIVIRUS UPDATED". I would pick AVG if they improved detection and brought the interface and update system into the 21st century.
Hope that helps.