I'm not sure whether Civ VI is the best game. I terms of gameplay mechanics, I would say no. The gameplay is very rich but many of the idea's (loyalty, world congress, religion, espionage) are poorly implemented and function more like salad-dressing to your chosen strategy of Great Work hogging / Space Racing / Domination.
And yet, I would say VI is
definitely better than Civ V, however. I've been replaying that lately (BNW with all the DLCs) and the contrast couldn't be clearer. Civ VI is, by far, the more enjoyable game by my experience.
Which is funny because I feel like the mechanics in Civ 5 work better overall? Religion works better, the world congress is better and the gameplay elements are easier to use by an AI (which I think is CIV VI's actual downside. The AI isn't worse in 6 than it was in 5, in fact. The difference is that the decision-making in Civ 5 is more straightforward than it is in 6, which is something an AI finds easier to handle than a plethora of equally valuable options. CiV VI's gameplay is, ironically, too rich for its own good.)
Anyway, the really big downside to Civ V has always been the early game. Progress is slow because you're not making culture (notable exceptions to this are Songhai, Poland and the Aztecs, the only civs with a meaningful, early-game culture boost), you're bleeding gold if you're not going tradition (in fact on huge maps, you can just give up if you fail to found a religion with Tithe unless you're playing a civ with a massive gold boost like the Inca) or if you're going early warmonger. Happiness is a crippling mechanic that punishes going wide, leaving large swaths of land uncolonised for the entire game. It's tiresome because you're just doing the same round of min-maxing irrespective of which civ you play.
The game offers you a way out with its exquisite social policy trees (which i definitely prefer over Civ VI's annoying card fiddling), as Aesthetics, Commerce and Honor can make a massive difference for the better, but the problem for me is really just the lack of variation in the early-midgame, which, ultimately, is the most interesting part of the game. You will always take Tradition and one of Aesthetics/Commerce and Rationalism. The final policy slots can go to any of the others (and sometimes you can squeeze out two policy trees if you're lucky and your ideology tree doesn't suit your interests).
By contrast, the late-game in Civ V is fun thanks to the artillery's indirect fire and the ideologies spicing up the endgame, and it's a damn shame I have to go through the same motions each time to get there. The early game being slow and tedious also neuters my interest in any civ with an early game UU (most notably Assyria and the Huns, whom I've never done well with) because I never find war to be worth its opportunity cost
On top of that, most of the CUA's aren't good and the few that are, are ridiculous. Japan and Denmark's bonuses are very hard to make use of, it's almost as if they don't exist. Civ 5's Portugal is perhaps the dullest Civ in the entire franchise (appalling city list too which racks my OCD each time they settle more than their Core Four). France stands out as a
pathetic joke, going from the best Civ in Vanilla to easily the worst Civ in BNW. On the flip side, Koreaand Poland are laughably broken, as are the Inca (which are an A-tier civ in *every* iteration), as are Songhai if you figure in their very nifty Mud Brick Mosques: +2 culture per city for an early-game building you can get at half price if you take the piety opener and its culture yields stack with Honor's +2 and Liberty's +1 (which you are encouraged to take as Songhai since money is less of a problem) and ofc the religious building of choice.
(No negative mentions of Venice, which is a special snowflake deserving of our love and affection. Don't blame Venice for being bad, Venice tries its best and I will happily share this screenshot of when I won my first and to date
only Civ 5 domination victory, as Venice, on a huge Pangeaea map, King Difficulty. Enjoy
(I had to deliberately throw a dip or culture win to get it but, toe-may-tuh / tuh-mah-toe)
I find it ironic that Civ VI took away all of the tedium Civ V had but replaced it with a near-sandbox-like experience. All the systems that were meaningful and fun in Civ V are irrelevant and dull in Civ VI and vice versa. There must be some sort of middle ground here and it should be found if Civ VII is to be any better.
Overall, I think each Civ game had its ups and downsides. For all the praise
Civ IV has received and still receives to this day, I've never been that fond of it compared to most other people. Civ 4 was always too difficult a game for me, as I'm still prone to getting beaten on Noble difficulty in 85% of my games.
Now, don't get my wrong: I don't really care about winning and never have, but I
hate losing on terms that aren't my own. If Greece beats me in space race, then fine, I've tried my best. But if Greece shows up on my border, with a 30 unit stack of doom, despite spawning on the *other side of the bleeping huge map* with several weak enemies between myself and them, and then declare war, and then capture five cities, razing three because they cannot handle the cultural pressure and/or maintenance, and the same thing happens at least
twice in every
three games I play, then it becomes super easy to enter Worldbuilder and spam Modern Armor in my cities with reckless abandon. I don't think I've ever ragequit my Civ games as often as I have in Civ IV. I don't mind getting attacked if I neglect my military, but to me the Civ 4 AI's ridiculous bloodlust always felt like a huge middlefinger towards peaceful builders such as myself (and Civ IV is THE builder game in the franchise. Its terraforming and wide range of buildings and wonders is some of the best found in any strategy game) and that realization has bled into my opinion of it. What a sad waste of perfectly good infrastructure.
So, I arrive at my personal conclusion that the best game in the franchise, in my opinion, is probably
Civ III. Civilization 3 was simple and straight-forward, but still challenging enough to meet a satisfying end. It never really got dull since late-game warfare was a thing ever AI happily engaged in and so did I since there's little else to do. The only real problems I had with it were, well,
form-related. The civ colours and city lists, both of which can easily be modified, which I have done. The game itself is a breeze and remains addictive and replayable. It is the only game in the franchise where I really feel like playing One More Turn after winning (or in Civ IV's case, being beaten).
It's definitely between that and VI.