In my opinion, Civ1 was fundamentally simpler than any other Civ game. It is like the difference between playing DOOM and Halo. Civ 1 has very few units, very few civ types, very few anything really. That means that it is easy to keep the whole game in your head at once. For me, its a totally different experience.

It's simple (both in terms of gameplay and graphics) and it's the fastest Civ game to complete a full playthrough. Later releases made the game slower and more complex.

I played a lot of Civ1, Colonization and Civ 2. First time I tried Civ 3 I lost some city due to some culture or religious influence and ragequit (I was also working my first job at that point so didn't have as much time to spare).

Played a bit of Civ 4 and 5(or 6?) but never was really as hooked on them.

1. It was the first civ.

2. The Settler unit was a big eared bat

Honestly it feels to me that Civ1 - Civ2 is the most direct upgrade in the series. Civ 2 was mostly just a better civ 1. From civ4 onwards, the series was a lot more willing to shake things up in its gameplay.