I prefer Norton, its never let me down, although it can be a bit pricey. I usually look for OEM's or upgrades. Systemsworks 2k3 OEM can be found for $20 online. What kind of computer is this? What purpose does it serve? Who has access to it? You might not need 'the best' antivirus if you just surf and play games. Especially if you rarely get a virus or trojan. (like me)
As for a free antivirus, I tried AVG, and was sorta dissapointed. Looked cheesy and found a trojan, but would never get rid of it. I finally switched to Avast Antivirus, and it's been good so far.