First of all, what do you mean with "best" - catches the most attacks, does not crash, is least drawing on resources, has the most features, is easiest to configure and use ...?
I have an old 266MHz Pc on which I used McAffee for several years until it slowed the system down to the extent that it was not usable anymore. Then I switched to Panda AntiVirus which was fine, but when I upgraded to the most recent version the new engine would catch so many apps as hostile and was simply so annoying that I had to get rid of it. Now I am using PC-Cillin 2005 together with Spybot on this old machine and it seems to keep things running well.
I also have a fairly new laptop with an AMD64, I use both Norton Internet Security 2005 as well as Norton SystemWorks 2005 on this machine. Setup was a bit difficult because bot apps share the same AV and I had to uninstall and reinstall a couple of times until the AV was smoothly integrated with both products. Since then it's running well and I can't complain about any intrusions or performancre problems.
I'm posting in this forum because Norton already has a duplicate Cleanup and Cleansweep features, and I was wondering if there was any added benefit from using CCleaner as well.