I'll back up that Civ 4 has been the best entry in the series so far.

Civ 5 is when they dropped unit stacking, which made combat much slower and more finicky since you couldn't just build up a massive deathball and tear across the map, and Civ 6 doubled down on that design space by tying city upgrades to individual tiles as well. They're not bad changes, and they do add more strategic depth to the combat and city-building, but they do make an already slow game substantially slower, since combats that used to be done in a turn or two now require several turns of rotating and repositioning units to get them in and out of the fight.

Civ 4 was the last "pure" civ experience, building off and adding to the previous games without sweeping mechanical changes to shake up the meta.