The best army is civ 3. No doubt about that. Civ 4 General thought gives special perks to 1 unit and is also nice.
But IF city center HP get removed, thus only unit HP count for city defence, so Ai would actually care about building a defence force
civ 3 mixed armies and generals (ultra elite unit, or champion, generates one), is the best possible defence av.
If you play Napoleon scenario in civ 3, you would understand what I mean for strong armies.
Roads and railroads as in civ 1
Partisans, Economy and unit cap tied to city pop as in civ 2, plus the best complex tech tree.
Mixed armies and governments as in civ 3, plus building on mountains.
City centers as in Civ 4 (city walls give a buff to unit inside but no hp by itself)
Realistic graphic as in civ 5, plus naval combat.
Ley lines, Heroes, workers sacrifice, but may need to mantain a little more automatic, or help build roads, etc. as in civ 6
Civ 7: Squash the water, bigger rivers, early armies, no city hp, 9-10 altitude layers, sequoia giants, giant wales, roaming livestock for nomad gameplay...and less cards gameplay... maybe a little sim like where you can name citizens and have mini missions, like driving a car like simcity2000, move citizens around cities, industrial districts, hospitals strikes... would be super fun to actually get inside a 3d view at ground level...
I would move all Gunpowder units into a a multi- function unit. In other words it can be both a melee(using a bayonet charge function) and ranged unit. Tank units should be moved to a special ranged and siege unit.
A diverse unit squad: This would be group of linked units consisting of Melee, Ranged, and Siege units. And later maybe adding a helicopter or a plane to that group. This would be an ideal invasion force.
In civ 3 the best defence against air unit is an army made of flacks, and later SAM units, but air and naval units did not have the possibility to form formations or armies. That would be super cool to have air and naval generals
to make them. City revolution had planes that could go out for as long as their range could sustain them, then they had to land and refuel for a couple turns, but in the meanwhile, they could travel for two to four turns to far away bases. Very nice mechanic that should be implemented to all units in the game. Air refueling units would be supercool, may need some naval support, like the Nimitz aircraft carrier around that could provide range cover.