You really can't go wrong with any of them, depending on your tolerance for older game engines and older gameplay styles. The newer releases incorporate newer gameplay styles and graphical styles. I love classic CRPGs but I know a lot of people who cut their teeth on Dragon's Age who despise the clunkiness and outdated graphics. To each their own.

Some vocal long-time users of things tend to dislike newer versions of that thing, no matter what it is. Personally I started with Civ 3 until I tried Civ 4, and then it just felt outdated. I liked Civ 4 until I tried Civ 5, and then it felt outdated. I liked Civ 5 until I tried Civ 6, and then it felt outdated.

If you started with Civ 5 or Civ 6, I don't think you'd be missing out or disappointed with anything.