im gonna say ceasers legion.
As a whole, it seems every man and soldier is trained to fight over years, the ones that are better are promoted up.
Whereas the ncr as a bulk as recruits that are semi trained over a period of months. Yes the rangers are an exception to this but also the rangers are a division of their own and dont make up the bulk of the army but a small percentage.
The battles between the ceasers legion,and ncr, mainly the times the ncr won thier battles was because they were already in a stronghold and defending it, which in battle alot of times gives the upper hand in battle.
The mwbos and the ncr and the legion have won battles against each other, but when an untech type army is able to win battles against people in pa suits weilding high tech weaponery and against armys that are civilized and have but not always supply their troops with advance armor and weaponery against savages mainly weilding machetes and rifles wearing football outfits, iy says alot about the power of that situation.
To be frank when we use real life examples of war world 2 of europe countries invading northern africa who are armed in melee, bows, and small arms and roll over them mainly due to superior tech and in this universe u have savages that are in the lowend of the techh department holding its own against superior tech and armor, its says alot imo.