Everyone has their preference when it comes to the AV group (Avira, Avast!, AVG) and I prefer Avast!
That being said, the best anti-virus is common sense. No anti-virus will protect you from something you download willingly against its wishes. Don't download plug-ins from random sites. Don't even open suspicious sounding sites (for your naughty inclinations). I would personally advise against all torrenting, but some people do it without getting infected.
Basically, just be responsible. Although if you really want to take the paranoid option, you could run everything you download through a sandbox program or even a virtual machine.