Gathering Storm. First and foremost because of the infrastructure changes. Power plants, canals, tunnels, railroads, etc are outstanding additions. Then because the included civs had the most unique playstyles, then because of the way disasters make the map dynamic.
RF was perfectly serviceable, and the loyalty and era mechanics were welcome additions to what had been a pretty static game before.
The others are all fine, and I will always welcome seeing more diversity in my opponents. I have found the game modes to be interesting in concept, but not always well executed.