Before AVG 8 came out, I would say AVG because it was quick at searching and stopped all viruses. Plus, it didn't slow down my computer. AVG 8-9 have been slow and bloated, they drag down my computer with them, they take over 6 hours to scan my computer, my computer usually crashes while scanning, there are 2 minutes (not exaggerating) between when I click the icon and when it opens... ew!
ThreatFire is definitely the best. Although it doesn't fully qualify as antivirus software.
EDIT:
And they're both free.
EDIT:
Oh, and I use AVG free, never had any false positives.