I actually vote for not getting an anti-virus at all.
Just be careful while browsing the internet, and you're good.
No need for a CPU and RAM-eating program to ruin your experience.
As long as you watch out, you'll be fine.
But if you really want one, then I suggest Avira, which served me well. Note that if you're getting the free version, you'll get an ad in a window by itself (which you can simply close) every couple hours or so. But it's really good.
Much as I would like that to be the case I can not agree and will have to call dangerous advice-
Advert networks have displayed malware before
There have been vulnerabilities in web browsers and plugins/extensions before (see anything made by adobe for more)
There have been vulnerabilities in sites before that allow them or part of their code to be hijacked before
There have been sites that have gone bad thanks to rogue admins before.
Sites have unintentionally uploaded malware before (one of the uploaders say had a virus and that attached to a file they uploaded).
And it goes on and on but those five are quite common. Occasionally you even get something novel as well.