Xeshra: They are not really tied to Steam, as an example Civ 6 is available on EGS, same for Civ 7.
Due to these two posts, you have instantly become one of my favorite people on this forum. XD
I have never played any Civ newer than IV because to my previous knowledge they all required an internet connection at least for activation. Since I do not have internet at home, this was never an option for me unless I was willing to haul my entire desktop to someone else's house long enough to activate. I mean, possible, sure, but annoying as hell. This is why I buy from GOG, offline installers all the way. But until this thread, I didn't know I could do the same with any newer version of Civ.
I would still much rather buy it from GOG, but if that's not possible, you've presented an acceptable alternative. Luckily for me, my ancient computer does meet the minimums for CivVI, though not by a whole lot, so I'll probably have to play on lower settings and smaller maps until I can upgrade. But that's better than not playing at all! :D
Catventurer: The Civilization games really did peek with Civ4.
eric5h5: That's the last one I bought, as I was getting kind of burned out on them even though Civ 4 was the best.
I have heard many times that CivIV is the best of the series, and it's easily my favorite of the ones I have, though I will also go back and play CivIII from time to time. Each version has their own charms and annoyances. But I have so badly wanted to try any of the newer versions just to see what they're like, which is why I'm so glad for Xeshra's posts. Even if IV ends up remaining my favorite, I've been wanting to try V or VI for years. Along with SimCity up to and including 4, Civilization is one of my all-time favorite game franchises, and it has irritated me for so long that they went to requiring internet to activate them.