Not sure on the first two. Need to see some stats on the Enterprise-D and Star Destroyer--offhand I think the Star Destroyer's a heavier ship but I'm not sure.
"Legionnaire vs. Hoplite?" The legionnaire's main advantage was in group tactics rather than one-on-one superiority, so my answer would depend on whether we're talking about two guys fighting or a whole group. In a group, I'd definitely pick legionnaires, given even numbers. In one-on-one fight I'd give the individual legionnaire a slight advantage due to superior weapons and armor, but the fight could go either way depending on the individual--the Hoplites were great athletes.
"USS Monitor and CSS Virginia ("Merrimac") vs. Spanish Armada?" I see this as a tactics issue as well as a technology-vs-numbers issue. The Armada with its numbers and maneuverability can surround the Monitor and Merrimack and cannonball them from all sides, ram them from all sides, and board them from all sides. To counter this the Monitor had 2 big guns and the Merrimack had 10 cannons and 5 guns, so given that I don't think the Monitor and Merrimack could sink the whole Armada quickly enough to survive before getting rammed/boarded even if they could take a lot of cannonball fire. One armor-piercing hit to the Monitor below water level could kill the whole crew.
So what's your picks?
You took me seriesly! :) But OK--
I view the Star Destroyer as having the mass and maneuver characteristics of a Borg Cube, and therefore the dynamic of the battle would resemble that between the Cube and several starships including Enterprise at the beginning of "First Contact". The large number of turret-mounted particle-beam weapon on the SD might provide an initial advantage, but if the shields could be disabled (by destroying the exposed shield generator) the turrets could be shot off.
If Easy Company were dug in on Balaclava Heights, the battle would have gone no better for the Light Brigade, IMHO. The rapid-fire, highly accurate weapons of the WW2 era would have made up for the smaller force.
I agree with you re the Romans and Greeks.
As for the sea battle, what would you make of the advantage in speed and maneuver provided by the steam engines? Would Captain Aubrey's "weather gage" be rendered irrelevant? Were any Spanish guns of 1588 capable of penetrating either the armor or the water (if aiming below the waterline)? I don't know.