Levrone had thickness hard to beat in the MM, so the Maryland Muscle Machine's Most Muscular was almost certainly more than 21" like that stupid Danimal list says.
Ronnie had way more peak and detail, but Levrone has the sheer thickness (esceline or not). In the front double Bi Ronnie appears to have bigger arms though since he has the shape and Levrone's go flat from that angle.
Vic Richards vs Paul Dillet would be an interesting one, but Vic never competed properly so I say Dillet had the most freaky looking arms, even if Kovacs' were technically bigger.
Ronnie 2003 has possibly the best arms, since the detail and separation is insane. Priest and Oliva win the forarms award, but that pic from the 2004 press conference where Ronnie does a front double bi in that blue top is the best arms have ever looked IMO.
Now Arnold in '72......especially considering it was the 70s and he's a tall guy, he had crazy huge arms!