Enterprise-D vs. Star Destroyer?
Legionnaire vs. Hoplite?
USS Monitor and CSS Virginia ("Merrimac") vs. Spanish Armada?*
*Does technology beat superior numbers?
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?