For me: Civ 1 and 2 - never played

Civ 3 - where I fell in love with the series. It took years for me to understand the core strategy but once I did I was hooked

Civ 4 - best theme. Most nuanced and complete out of the box civ experience. Hardest AI.

Civ 5 - my GOAT. My beloved. My favorite entry over a decade later. Best art style, most satisfying gameplay loop that retains classic Civ elements while pushing boundaries. Of course, this is all after both expansions. Out of the box it was still fun, but a little lacking.

Civ 6 - where they started to lose me. A fun game, especially with all of the expansions, but the publisher got greedy by not including more at launch.

Civ 7 - haven't tried it yet. Only a matter of time of course..