Civ4 was highly flawed IMO, because the tactics revolved around setting up giant "death stacks".
There were a few units that could attack a whole stack, but ~20 musketeers in the midgame was practically impossible to defend against aside from having your own stack of ~20 musketeers.
Civ3 and Civ2 had the right idea: Kill-stack the entire stack if you group your units together like that. There was much richer play of combat in Civ3 and Civ2. It was non-intuitive, and players complained grossly about kill-stacks... but lets be frank: the game was better with kill-stacks.
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Between Lenard Nemoy as the narrator and the incredible music, I'd think 4 was the best production value of the entire series however. Civ6 comes in at 2nd place, pretty good music and Sean Bean is pretty decent too. Just not as good as Civ4 IMO.
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Civ6's issues is the "flattening" of the entire tree. Nothing feels different until you reach airplanes. Riflemen are just reskinned Musketeers, who are reskinned Swordmen, who are just reskinned Warriors.
Tanks are just reskinned Knights, who were reskinned Chariots.
In Civ6, there are basically only the following units: Anti-city Siege (Catapult / Bombard / Artillery), Warriors (Warrior, Swordmen, Musketeer, Riflemen, Mech. Infantry), Heavy Cavalry (Chariot, Knights, Tanks), Light Cavalry (Horsemen, Cavalry, Helicopters), and ranged-units (Slinger, Archer, Crossbowman, Field Cannon).
Eventually, airplanes take over the siege and ranged roles (Biplane / Fighters ultimately play like faster Archers, while Bombers play like better Catapults). But otherwise, there's not much difference to combat through the ages.
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Furthermore, Civ6 doesn't allow any exponential growth strategies. Aka: Wide vs Tall, or whatnot. In previous games (Civ1 through 5), buildings provided multiplicative bonuses.
So Library was +100% science for the city (or some other multiplier, depending on which version of the game). This means that Libraries had a "threshold", where they were useless for small cities (+100% science on 1-Sci base production is just +1 science), but powerful for large cities (+100% off of a city with 20 science is +20 science).
But there's none of that "play" in Civ6. Its wide-wide-wide for the win. Sure, Civ6 has increasing costs for settlers / builders to try and make the "wide" strategy weaker... but that just slows down the player. It doesn't actually change the fact that wide is the only way to play.
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Builders only can make 3-improvements before they disappear (unless you have Feudalism or Pyramids: increaasing the limit by +2 or +1).
In contrast, Civ3 through Civ5 had Workers (and Civ1 / Civ2 had settlers) that can continuously upgrade your city throughout all the ages. Workers vs Military was a key balance throughout the game.
While in Civ6, its basically "Builders until they become too expensive", and maybe "Play Feudalism Card every now and then to cheat some extra improvements out of your builders". There's no depth to the decision.