It's a mixed bag for me. Each of Civ IV, V, and VI have things that they do better than the other games in the series.
Civ IV still has my favorite tech tree of the series. Most techs have optional prerequisites, which means you are more free to focus on only the techs that you want. There were some "dead" techs, but they were mostly leaf techs that weren't necessary for progressing further into the tech tree.
Civ IV also had permanent alliances and vassals, which are both concepts that I would like to see return to the series in some form. The lack of a cooperative victory is, IMO, one of the biggest things holding diplomacy back in Civ V and VI because no matter how peaceful you've played, you can't work with the other civs to achieve victory.
As for Civ V, I actually preferred how that game handled both trade routes and great works (in BNW, of course).
I liked that trade routes in Civ V have reciprocal benefits right from the start, so that receiving a trade route from another civ actually benefits you and gives you a reason to maintain peace with that civ.
I also liked how great works were more relevant earlier in Civ V, and getting theme bonuses was easier and more common.
I feel like both of these mechanics regressed in Civ VI.
As for Civ VI, I love how much more relevant the map terrain and geography feels (with the district adjacency mechanics and so forth). The unstacking of districts and other visual flourishes pack so much more information onto the map (compared to Civ V). You can see every piece of infrastructure in a city from the main map screen. Every improvement has a distinct graphic or animation for when it is worked versus un-worked, so that you know (just by looking) what tiles are being worked and which aren't. I wish Civ VI also had a variant of the little cabin that appeared on unimproved worked tiles, but meh...