I am playing Civ 7 more than any other game in the series right now due to its newness and it's monthly updates. Currently, it is the game in the franchise with the most potential. All other titles have reached whatever potential they had with Firaxis and now mods alone are their only way of tapping in to more potential. So Civ 7 holds an edge against the competition currently for those of us curious about the new direction of the series that each game held before it. If Civ 7 were to completely stop all development and updates I suppose by this Feb, the Civ titles I play most often would become:
1. Civ 4
2. Civ 7, Beyond Earth.
3. Civ 6, Civ 3, Colonization.
4. Civ 2 and CtP
5. Civ 1 & 5
Group 2 are both games I think would get occasional play from me but suffer from a lack of variety in gameplay. The gameplay 'scratches an itch" but is mostly a 1 trick pony with enough variation to make a game or two feel unique. Civ 7 offers some gameplay variety with uniques but gets bogged down by legacy paths and very predictable diplomacy. As the game goes on both of these become more pronounced and strict until the gameplay stops feeling dynamic at all. Modern uniques almost feel irrelevant. 1.2.5 has helped a little with variation but only a little. It still dulls out by modern. Beyond Earth gets fair playtime from me because of its sci-fi flavor. Similar to 7, the early game offers something in variety and pursuing affinities to customize your military and playstyle is fun. But as you start reaching the mid game your course is locked and diplomacy is predictable making the gameplay no longer feel engaging. The stakes get lowered, not raised. Both of these are great for their specific flavor of game they offer but 2 playthroughs easily make you see that the 3rd playthrough will follow a very similar path. Luckily, Civ 7 currently has a new build every month offering new features in your my next playthrough. 2 games is the most I can get in a month and I usually abandon my 2nd game to start fresh with the new build.
Group 3-5 are games I play to reminisce more than anything. They too either scratch an itch or I play them to curiously reinvestigate old mechanics. None of these get played very often. If I want to play Civ, Groups 1 and 2 appeal to me the most. Group 3 has a very small chance to tempt me due to some special mechanics I enjoy that are unique to them. All 3 of these games appeal are exclusively due to mods. Groups 4 and 5 just do not appeal much anymore at all beyond nostalgia. I do plan to one day try out the famous Vox Populi mod for Civ 5 but it does not intrigue me very much at all.
Group 1 - Obviously my preferred choice though it currently is taking a backseat to 7 because I have seen everything it has to offer - whereas with 7, this is not true. Civ 4 is going to be hard to topple because it feeds me the nostalgia but is also incredibly flexible due how versatile food and commerce both are in its system. I have a variety of mods that tweak the formula of historical strategy, as well as FfH2 to have a fantasy setting, and Planetfall and Dune for sci-fi. The system is reminiscent of the early games but offers the best of all of them in my opinion plus it holds it own unique mechanics that I mostly enjoy. This title can give me personally everything the rest of the list offers all put together. It is possible for Civ 7 to make it to group one, but it is so unique that Civ 7 making it to the top would not push Civ 4 down because they are so different. Civ 7 cannot offer nostalgia based on Civ 1-3, Civ 4 magically somehow does.
I hope Civ 7 can climb the ladder one more spot, but I don't see Civ 7 dropping below group 3 ever. If it ever does fall to group 3 for me, it will probably push Civ 6 down to Group 4. But I suspect it will always be in Group 1 or 2 after further development.