I like NOD32 the best. Reasons.
1) It has the highest virus detection rate.
2) It runs fine in one of my old PCs (533 MHz, 184 MB RAM) where KAV and even BitDefender cause problems.
3) Its heuristic detection method seems to me the most effective.
4) It does not update each hour (I suspect that programs that do that are using placebo effect most of the times), but reacts rapidly to crisis. For instance, during the last crisis, some days ago, it updates 6 times during a specific day.
5) It has the shortest scan time.
6) As far as I know, ESET, NOD32's owner, is devoted to this antivirus program ONLY. I believe that fighting viruses is a full time job, and that Champion_Munch is right when said that: "... program that will claim to clean everything often it has poor detection rates".
7) Is has no co-existence problems with any other of my security programs (Spy Sweeper, Ad-Aware, SpywareBlaster, BlackICE). Other anti-virus I have used had this kind of problems.