Played all the Civs, three of the Call to Powers, SMAC, Alpha Centauri, Master of Magic, Endless Legend, Master or Orion 1, 2, and CtS, and GalCivs 1 and 2.
I liked nearly all the Civs. I liked IV just fine, but I don't have the rose-tinted glasses many veterans have. When that game released, the AI was so bad you'd have undeveloped land in the 1600s. Large tracts of undeveloped land. It simply didn't know how to use its workers, so it was trivial to defeat even at the highest levels. Even after a few patches, the unit balance and SoDs were all over the place in terms of balance - a lot like Civ V, actually. Civ IV and Civ V were very similar. Arguably, Civ V matured quicker, because it was mostly serviceable after Gods and Kings, whereas IV only really became good after Beyond the Sword. Even then the diplo of the AI was so manipulable that the higher setting was basically just a diplo game to be able to use the AI advantages in your favor. It's one of the chief reasons why tech trading isn't in Civ anymore. Civ IV showed us how truly broken it was.
I've played about 6-7 hours of Civ 6. I like what I see. It's the most fully fleshed out Civ at release. It's so fully-featured that it's actually kind of intimidating. I'm used to the Civs ramping up over two expansions. Here, they hit you with everything right out the gate and then some. This ain't no simple game. It's arguably the most complex building game out of all the Civs to date. The graphics reflect that. Some people say that they're simple. They are. They're supposed to be, because they're iconographic representations of actual functional data. You can tell what any city is like and how it works just by looking at the tile improvements and districts. You don't need to see a Building List. It's all on the map in graphical form. There isn't a City View. You don't need it. All the information is on the overland map.
I confess that I'm also a fan of Civ Revolutions. That's a much simpler game that recalls Civ 1. Some ideas in Civ Rev are in Civ 6, but all to the better. Armies and Corps are limited same-unit-stacking. It's kind of like Civ Rev. Civ Rev actually implemented it a bit like it was in Civ 3: Warlords, and Civ Rev showed that it wasn't as broken as feared. Civ 6's Army is even less intimidating than that, so you still stand a chance against an army, but there's a benefit to it, too, if you have that many hammers.
Roads are Civ Rev-ish. Civ Rev does away with building roads as consuming busy-work. You buy roads from a menu, then it's built. It's a great idea. Civ6 kind of does it by assigning road-work to Trader units - like a trade unit and auto-road worker in one. You can eventually ad hoc roads with Military Engineers. Roads are different in Civ6 in that they DON'T add movement points to any unit. They just allow your unit to move normally over difficult terrain. Handy over Jungle and Hills. Less useful over Plains and Desert.
On the whole it's actually an incremental improvement over Civ:BE, building on ideas in the much-maligned side-sequel to Civ V, adding some ideas from Civ Rev and some from Endless Legend and Magus. If I make it sound boring, let me be clear that it's not. I think it's the best Civ at launch I've ever played.
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