AV software depends on what you what.
Do you want something that runs continously consuming resourses and checking every file you access constantly?
Or do you want a simple small program you can use just to check new downloaded files manually before opening, and maybe run a full scan once a week?

I use to run AVG and avast, but I didn't need that level of obtrusiveness and memory stolen from me, as I didn't seem to get any viruses anyway. I didn't need every program checked on schedule or every email checked.

Now I just use clamwin, it's a manual scanner that doesn't protect you automatically, so it doesn't hog you PC memory. You just manually right-click and scan any downloads or attachments before opening and if you know what you're doing, that, a good firewall, good malware scanners and good PC usage is enough.
You don't have to pay continually for a product or updates (clam win is free and has free updates), and you don't need some resource hogging bloatware always running on your PC.
I recommend avast if you want something fully automatic and have a decent spec PC though, small enough and funtional enough.