Was the Roman Army the most effective fighting force pre-gunpowder?

[Frank]

You might be quick to point out that the Middle Age Europeans had the edge here, but did they really?  Yes, they hate advanced plate, stirrups, and more weapons... but only a small portion of them were such equipped.   

The equipment of a roman equites of the principate and a medieval knight (e.g in 955 or 1066) is pretty comparable. Medieval plate armor was introduced rather late. We should compare this late medieval knight with a roman cataphract. Again not that much of a difference regarding equipment. The difference is more about cavalry tactics. 
So with a comparable cavalry, superior heavy and light infantry, overall way more numbers and better logistics, the roman army at its height (100 BC - 200 AD) was most probably the most powerful and efficient western army pre-gun powder. But this is primarily based on numbers and military infrastructure.