This is a question that has come up here and there but hasn't really been discussed much.
Based on in-game eventsand lore, who are the strongest fighters? Mind you, in-game statistics aren't neccesarily reliable since for example NCR Rangers will destroy Centurions but go 50-50 with Praetorians even though Praetorians are nothing but hand-picked Centurions to be Caesar's bodyguards, so try to avoid using those.
I'll start by simply proposing this list as a possible answer. This list is what I think could POSSIBLY represent different soldiers and their respective strength, starting from the strongest and ending at the weakest:
Enclave Remnants
Legate
Veteran Ranger
Centurion
Brotherhood Paladin
Super Mutant - Deathclaw
Legion Veteran/Decanus/Prime
Ranger
Securitron Mark II
Brotherhood Knight (or other)
Legion Recruit
NCR Trooper
Great Khan
Powder Ganger
Not claiming the list is perfect, I'm more or less just curious in hearing how other people interpreted the relative strength of each unit based on lore.