Size and ability to kill another fish really has little to do with being "mean". A mean fish to me is one that attacks any other fish unprovoked and not while breeding or guarding fry. A mean fish could attack another, larger, better equipped fish and get eaten. Therefore, I'd have to say the meanest fish I've had was an istlanum. My male triamc is really mean too. My hait and my odo are still too juvenile to know how mean they are, but they have both killed tankmates as juveniles. I'd say a mean fish is one that attacks it's owner as well, and not when protecting fry. You don't see many doviis attacking the owners unless they are guarding fry. Midas, trimacs, istlanum and others I have seen do this. I think Aquamojos great Hericthys carpintis "Esondido" would bang the glass to get at the owner.
I just think it is important to separate "mean" from killing ability. To me, "mean" is a personality trait.
Stepping off soap box now.