Another question, what 4x has the best diplomacy?
While not exactly a 4X game, more of a global political simulator, Rulers of Nations(Geopolitical Simulator 2) had some great mechanics in the social aspect. You could easily start with one of the most powerful nations and then try to attack and occupy any nation. Without a good provocation that your people would believe in, your approval ratings would freefall from being at state of war, getting sanctions and becoming isolated from the world. Even if you start with North Korea this would happen and you'd be thrown out of the office or assassinated.
Trading was strange, as you formed trade treaties even though a nation doesn't own all products that are produced within it.
Everything else in that game sucked though. Changing policies gave no insight as who would be for it and who would be against, bringing up protests from a group you didn't know existed and easily get thrown out of the office. Every week your wife/husband would talk to the media and lower your approval, with the only option I saw to deal with this was assassination.
Economics were badly simulated. To test this I cheated to have a 100%/10000% tax on pornography. Payments were still coming in so I guess they still prospered, the black market must have been bigger than the drug market. There is also the bug of subsidizing industries with billions/trillions of dollars for just one day, making no changes to the budget and have a super industry pop up in a week.
It had too much micromanagement for armies. It would have been better if you were seeing and controlling gradients of land, naval and air. Any nation would surrender after a day their capital was occupied. So a tiny nation with black helicopters could easily overtake any nation by outmaneuvering the slow AI.
What's worse is that the developers made a new game without fixing the previous one, no way am I trying a new one for the same price.