All software is free if you know where to look. anyway i would say MacAfee since they have a built in all-in-one center with antivirus+antispy+antiad+site security
basically that security center protects you from all downloads or threats on the internet and it even tells you in google searches which sites are safe and which arnt. also if you want just a separate good antivirus use Nod32. i've heard a lot of good stuff about it. or karparsky.