Let’s get this one out of the way first: Civilization is the granddaddy of 4X games, and you can’t go wrong with it. You take control of a famous civilisation, taking them from the stone age to the modern day; all the while expanding your borders, developing your cities and dealing with your enemies. You can win by playing to your civilisation’s advantages and using your terrain tactfully, choosing to either dominate your opponents with force or with culture and diplomacy.
Civilization V (and both its DLCs) is without a doubt the best in the series, and holds up today as a shining example of what a great 4X game should look like. The most recent entry, Civilization VI, is worth playing too, but it feels inferior to its older brother with less tactically variety and slightly less balanced gameplay.