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 machin