I'd vote for 6. 4 is the other "local maxima"; IMO 6 is a strict improvement on 5. They essentially just matured all of the mechanics, and then have added some

I'd vote for 6. 4 is the other "local maxima"; IMO 6 is a strict improvement on 5. They essentially just matured all of the mechanics, and then have added some new ones that aren't quite there yet (civ 7 some day...) (Although I do miss single-city runs as Venice in 5...)