4 is the peak. 5 and 6 are fundamentally flawed by their unit/movement system. Since you can only have one unit per tile, armies take up a lot of space and can't really move around well. To ameliorate that problem, they reduce how many units and cities you can have, to the point where it doesn't feel like you have a whole civilization under your control, just a traffic jam.

Civ 4's designer made Old World. I would check that out.

I started playing 2 like 25 years ago. 3 was obviously a huge upgrade, I liked 4 better but it was really similar to 3 but a little more in depth and shinier. 5 was quite a bit different, they went to a new map grid style, and changed up a lot of stuff I cant remember it all because its been so long and I havnt played 4 or 5 in forever. I didnt enjoy 5 as much, it felt like you had to play tall, and spam wonders no matter what.

6 has been my favorite out of all them. They added districts which you have to build that are effected by terrain and improvements, they also unlock certain wonders. It made positioning them critical and long term planning is way more important that any other of the titles. The complexity of it really made it so you had to play the game to each map. You cant just go in with a build order in mind and expect to do well like you could in 5.

That being said I dont see how they are going to improve on it very much. 6 seems about as good as it could get. If they make it more complex you started edging in to EU4 territory which the complexity of it can be a drag to fun for a lot of people.

Exactly. They are all the same game.