The Roman Empire
Ottoman Empire(15th and 16th century)
The Mongol Empire
Very big into roman history and I'd say them. Someone else has already given a great explanation on why too, their military was very professional, very disciplined and they were more organized than their opponents. To take the romans and Persians: the Persians easily outnumber the Romans but they lacked training and good equipment. 30,000 romans could definitely put up a good fight vs 100,000 Persians just by their tactics, discipline and superior weaponry. One of my favorite things on roman discipline is a legion was punished by getting beat to death if they fled a fight. 1 out of every ten men were picked to be beaten and the other 9 were given sticks to do so. This instilled fear of disobeying your officers in battle to where the romans feared their own more than the enemy. Another favorite thing was the legions themselves. You had the defensive testudo formation that made them a moving shield, imagine being on the opposing side and seeing hundreds of arrows not even denying the roman advancement because of the formation. This is what many warring factions lacked and why the romans prevailed just about every time. Furthermore the romans were setup in tiers (princapiis, triarii, auxiliary, etc etc and I'm probably spelling them wrong but each tier had their own classification) where the youngest and most inexperienced fought first, as they tired, they fell back as the next tier took their spot and they'd be fresh and more experienced as the cycle progressed, it would just be a repeating process until you cycled back through. Most of the great generals also kept a reserve unit for emergencies. They'd also have the better equipment as the longer you served and the more money you earned, the better armor you could afford. I think it would've been awesome just to see what could've been had Rome not fallen. Romes issue was being so big that eventually their military just couldn't keep up fighting on so many borders as more factions started warring vs them, you could add bad leadership from corrupt senate to just bad emperors, romes treasury slowly depleting (which was a big issue as the military relied on money), etc etc. A lot of modern day things are influenced by what the romans did in their time.