I'd have to say Civ 5 just from my playtime. The incorporation of alot of features, gameplay, graphics, playable nations, and all around great game.
But there's a lot of aspect I miss from the older civs, I do miss having actual spy units. I still think the privateers should be able to enter other nations borders regardless of treaties like in the older civs. And that upgradeable throne room.. lol yeah actually do miss that. Something about going from a cave with animal print to something modern to greet your ppl in is nice. I think especially with the higher focus of multiplayer in today's industry this could be a interesting feature to bring back down the line once other more relevant game play/multiplayer quirks are ironed out.
As for old school Civ genre games instead of just features, I seriously enjoyed Alpha Centauri. It does warm the heart to see so many others voice the same opinion.
I'm looking forward to Civ VI and am expecting it to top Civ 5.
Will Civ 6 meet our expectations? Can it stand the test of time?