Leathaface
Emperor
Civilization IV is the richest and most complex of the Civ games (with the toughest AI to beat and the highest difficulty curve), and the best diplomacy system (and not just because you can trade bananas and other resources that are now untradeable in V and VI). Also has the best main menu theme in Baba Yetu (vanilla Civilization IV theme). Also has stacks of doom (you can stack many units in one tile and move them all together as part of one army). Features Diplomatic victory which requires (in some cases) for the AI to vote for you, random events (in Beyond the Sword expansion). Excellent modability, which made for very fun mods ranging from fantasy (Fall of Haven) to sci fi to tactical zombie killing combat scenario (in the Beyond the Sword expansion).
Civilization V boasts a decent user interface and realistic graphics, less demanding learning curve, with the best leader screens, and excellent peace/war music for each civilization based upon famous national songs. Has carpets of doom, or 1 unit per turn, requiring you to undergo the tedium of manually clicking every single *freaking* unit to move them to the enemy (since you can't stack them). Terrible modability.
Civilization VI boasts a colorful graphic style, a day-night cycle, and complex civilization/leader abilities, though with less immersive leader screens than in V and a weaker diplomacy system than in IV, though with significant mechanics borrowed from V (like the 1 unit per tile), and some changes (siege mechanics, religious victory, etc). Terrible modability.
I think you can add Districts as a strength for Civ VI. Plus the way the cities aren't just one tile, but make use of its surroundings. (Adjacency bonuses, etc)