Imperial era Roman military prowess as opposed to those of the empires that sprung up after its fall.

Apples to oranges. Each empire's military was tailored to fight in the specific kind of war suited to their situation in a particular time frame.

Carolingian armies for example, were primarily either for smaller unit raiding like in the Saxon wars, or as part of a siege train to take fortified Italian cities like Pavia. This was as opposed to straight standing army on army combat, which was quite rare (with the notable exception of Tours).

Byzantine armies after the Arab conquest were primarily defensive in the early years and were heavily cavalry based as they sought to avoid head on confrontation with Arab armies in Anatolia.

Roman armies between the high empire and the late empire were optimized differently on the basis of not only tactics (offensive campaigning, vs. frontier defense vs. mobile cavalry to respond to incursions), but on the basis of politics. Namely, after the crisis of the 3rd century, emperors simply didn't trust field armies on the fringes to be independently better equipped than the armies which were around their physical presence due to rebellion.

Say you take these armies out of the situation they were designed for. Well here's a question, what situation do you want to put these armies in? Because there is no such thing as a "neutral" battlefield. Your choice of flat vs. hilly vs. rocky ground vs. cold vs. hot all have an impact as to performance. Not to mention what's missing from most of these "deadliest warrior" type matchups, the political factors. How "trustworthy" are these armies? Can you count on their loyalty and morale in combat? Will this differ between if they're fighting enemy states, barbarians, or competing factions in a civil war?

Not to mention the most glaring difference of them all. That even when you try to compare the applies and oranges of even medieval militaries to the Roman military, with the supposedly more advanced "military technology" of the medieval era, you can't get around the fact that the Romans were capable of fielding an overall standing army in the hundreds of thousands. Compared to medieval armies of kingdoms which rarely exceeded 20,000.

So what "size" do you want to place on the limitations of these armies? Because to artifically impose a number limit is to not be reflective of the "military prowess" you're seeking to compare.

Long story short, this comparison is going to be moot. Because the limitations you impose upon the scenario of comparison, will ultimately shape who is the winner because there is no such "neutral" ground for comparison, except that which you are arbitrarily going to impose.