Not sure what exactly the question is asking. I play King and sometimes Emperor. So I'll give my unit strategy for that.
I like to have 4 swordsmen (or 1 warrior per city + 1 extra and then upgrade them) early on. I like to build at least one ranged unit like an archer or mounted archer, or both. And 2 to 4 catapults. You can take over a nearby city by turn 60 or so even with just 4 swordsmen and one catapult.
So 4 front line melee units, 4 catapult type units and 1 or 2 other kind of ranged units. And I tend to have several such troops. Upgrade them through the eras.
For cities, look at the defense. You need melee units as strong as the city's defense before going in.
As for strategy, I don't keep all my melee units in front of catapults/cannons/artillery, etc. like most people do. I usually only keep one or two melee units up front for sight. With a mounted unit, you can move up front, use your cannons/artillery, and then move the mounted unit back.
So I'd say I upgrade catapults, trebuchet, cannon, artillery (this wins wars), and rocket artillery (this is an awesome unit). For melee units, I'd guess it goes warrior, swordsmen, longswordsmen, Riflemen, infantry, mechanized infantry. All of these are quite strong. Like I said, 4 upgraded catapults and 4 upgraded warriors and one or two ranged or mounted units no matter the era. I do use other units though. But I hate pikemen. dunno. They annoy me. In fact, I take them out with my mounted units and they're supposed to be weak against them. Strange.
Oh, tanks and bombers at the end as well. Stealth if you can (basically game over at that point). And nukes. I think they did a fantastic job with the animation and sound on that one. Actually, sometimes I just go for the win and don't go for nukes.
Still, nothing beats giant death robots. Those are just plain fun.
One last thing. I sometimes put a strong ship in my coastal cities that are near a border for extra defense. They stack with a land unit in the city along with a GG. So your city can actually attack three times from that one tile.