manfred was not a pro bber, but he probably had the biggest arms or some other guy with synthol.
seriously, as guys like valantino have proven, synthol has ruined the whole notion of biggest arms because all a guy has to do is put the most synthol in his arms and voila, biggest arms.
as far as muscular arms that i think were real i'd say dillet had the biggest, but then, his arms never seemed to get that rock hard look so i'm not even 100% about him.
bertil fox probably had the best combination of size and density and were outrageously thick for a pre synthol period. guys like fox and bill pettis had that kind of freaky development that looked as strong as they were big.
marko and marcus just look like they forgot to shoot the tris. that is probably unfair but these kinds of arms (where one muscle group ie biceps, just seems to jutt out disproprtionately to everything else) just don't have that much impact imo.
prime ronnie's arms, condition, size, density, detail, credibility seem to be the most developed in recent memory and are definitely the most complete of any mr olympia in the history of the sport.