I think my top ten 185 and unders would include the following (in no order)

Langford
Fitzsimmons
Tunney
Ezzard
Foster
Conn
Evander
Dempsey
Moore
Spinks

Now Rocky beat two of them, so while both were probably past their peaks, tought to reverse those.
Langford is where I think the rules under which they fight are extremely important. In a 25 round fight? As fit as Rocky was, nobody could sustain that pace that long. In a 15 rounder? Either Sam stops him on cuts or the Rock wins a decision. Fitzsimmons hung in there with Jim Jffries as an old man, but when Jeff turned up the heat Fitz was just too small. Same thing happenes here I think. Tunney is a toughie for the Rock. Could he have just stayed away and jabbed for 15? Same with Billy Conn. Tunney? Maybe. Conn gets stuipid and Rocky drills him late. Bob Foster and Spinks get blown away. They just didn't have a heavyweight's chin. Evander and Dempsey would both be "we need to have an ambulance in both corners" kind of fights. Just brutal, brutal affairs.

I think Tunney and Langford are probably the toughest style matchups and Evander and Dempsey would be gladitorial contests where I fear to pick a winner.

I will say that the more footage of Marciano one watches the more impressed one must be. Despite his rep for being awkward? he was amazingly strong, monumentally fit and while technically inconsistent? When he did things correctly? The result was a bolt of lightning. Watch the double shuffle that gets him to the right distance to KO Walcott or the double hook that destroys Kid Matthews.