Mion
Always Tired
Rome. The organizational effectiveness as well as the bureaucratically ran professional army would take out Westeroes fairly quickly. Especially coming off of the recovery after the victory of the Second Punic War. Them formations against the disorganization of a medieval army? :|
That's of course, with a strong leader(s) heading Rome of course. I had in mind Augustus at the helm or even Tiberius.
Westeros has huge variety though and I would believe they would outclass them navy wise.