The only free version I haven't tried yet is Bit Defender.
AVG: It's scan engine is quick however when I tried it, I had to do a Manuel update and when I detect the virus I had soo much time removing the virus, it wasn't even funny at first.
Anti Vir: Heck alot better and easier than AVG, I sometimes recommend this to novice users. But Anti Vir doesn't have a place to quarantine the infected file, it either repair or delete which sucks cause then you don't know if it's a false positive or not until it's too late. The updates are big and the server is always bad in the mornings (at my time zone that is).
Avast!: Unlike Anti Vir and AVG (which has one or two shields) avast! has at least 7 Shields which does take a reasonable amount of memory from your RAM. I love the Webshield and how it detects and
BLOCKS the virus from the website. Automatic updates (enjoy it) which I must say are small (wonderful). A few bugs but most of the time it's solve easily.
This is just my review base on my personal experience, but you can choose which antivirus that best suites you and you're allow to change your mind.