roy is the most evocative of the batch if they're used as they've typically been, since they're all pretty zero dimensional tropes but "johnny bravo" at least has any sort of cultural valence at all compared to idiot, goofball x 2, moron, stale bread, and girl
"mad scientist" is 2nd on the list, but ludwig probably has the most potential if they tried making the koopalings serious characters for some reason - you can think of a game where he tries to take the plot more seriously than bowser does for instance, whereas roy can't really not be the goofiest guy in the room

ludwig would be a fun villain for a game starring bowser. jr, for instance