I haven’t played it, but Civ IV seems to be the best version in terms of execution. However, a lot of the mechanics of the game are quite dated. I genuinely think FXS could do really well with a Civ IV remastered but I doubt we’ll see that. More likely would be an indy developer making an iteration of Civ IV. Unfortunately, so far the various “free civs” have only focused Civ 2 - 3, and every other 4X I’ve seen recently seems focused on Civ V.
Civ VI is, however, the current “best” iteration the franchise overall. It’s essentially “Civ’s Greatest Hits”, and basically builds on and improves everything that’s come before.
Civ VI’s weakness, however, is that it’s still not well balanced, it’s still missing a few mechanics and other elements from earlier games, and the overall difficulty is not well tuned at higher levels. Whether Civ VI will ever eclipse eg Civ IV really comes down to whether FXS and or modders can improve the execution. NFP is obviously an opportunity for FXS to do that, but we’ll just have to wait and see.