The short answer is I don't know,
But up in Oregon the Rusty Crayfish (Orconectes rusticus) seem to out compete the local crawdads, the signal crayfish (Pacifastacus leniusculus), himself not a wimpy specimen.
I dont know if red swamp crays, are the most agressive (how can that be definitively quantified? There are about 350 species in the US alone), mine would probably eat anything he could catch, however being a slow creature, he was constantly harassed by a Convict cichlid and Jack Dempsey, they eventually bit an eye off him. I would agree that they seem to squabble frequently and act agressively if you have a few of them in a tank together.
Also: In California the Red Swamp cray has beat a lot of species back toward the brink of threatened and/or endangered status, something to be considered. For Example: They maul the california newt and interfere with their breeding. (why other than out right territorial aggression? they probably can't eat anything that toxic.)
From my experience with crayfish, I would say size has the most to do with aggression. Larger species appear to be more aggressive. Based on that logic, see if you can get your hands on some of those monster crayfish from Australia or Tasmania and you probably have the most aggressive species.