I've been playing Civ since before it had numbers like many other posters here
I think we all have a habit of forgetting what those were really like (at release and after a while). The original had the advantage of nothing else like it being available, the rules were very weak and the AI was really, really bad.
Civ 2 was decent, but again had no real competition and the same problems of the original until a second developer made Call to Power which was exceptional (and in my opinion vastly superior to anything Civ 3 did).
Civ 3 launched without multiplayer support in the dawn of multiplayer games and you were SOL until PTW which, in my opinion, didn't make the game better it just made it so you could suffer through it with friends. I can think of nothing good to say about Civ 3 other than it released along with a lot of other good games so I survived.
Civ 4 at launch was kind of funny, a LOT of people couldn't even get it to run and many vocal members of the modding community were in an uproar with the change to 3D. Although Warlords was good Civ 4 was still just OK until BTS was released and then it was amazing. Mod capabilities in Civ 4 made it the best of the series with some really amazing and ground breaking mods (look at how many people bought Civ 4 just to play Fall From Heaven, it's insane). This was a hard act to follow for any game, not just another Civ game specifically.
Civ 5 is recent enough that most people didn't forget about it's launch. I still remember all the fighting over the changes to hexes and 1UPT (both of which I love as rule changes to the franchise). We lost everything that Civ 4 had evolved into and got a very basic game which was more difficult to mod than Civ 4. Civ 5 did have two very good expansions though, G&K was very well done and was a great improvement and I think a lot of us were surprised when BNW came out and felt even better than G&K. Even with two expansions though it still lacks features from 3 & 4.
That said, Civ 6 is a really good game. It is by far the most "feature complete" in the franchise at launch, not the bare bones game that 3, 4 or 5 were at launch. It adds more new mechanics than any previous Civ has (some of which are almost as polarizing as US politics) and it is already proving easier to mod than Civ 5 was in some aspects even without any official tools or pre-release SDK access to well known mod authors. The AI is not really worse than it was in earlier games but the combination of typical Civilization AI mistakes and easy human player exploits makes it look really, really bad. Toss in some sloppy mistakes not directly related to the AI (tech/civic tree dead-ends and shortcuts) and the problems are compounded further. The great news is that unlike Civ 5 this time we won't have to wait for expansions to get an amazing game, the AI, tech tree and balance issues are already being addressed by the modding community since it's far easier to adjust game mechanics and rules than it is to create them. If you've been around long enough to play the other 5 you're really just doing yourself a disservice by avoiding Civ 6.