UI Mods:
Better Deal window
Better Reports screen
Detailed Map Tacks
Enhanced Mod Manager
Happiness and Growth Indicators
Hillier Hills
Unique District Icons
Map Search Extension
Policy Change Reminder
Radial Measuring Tool
Real Great People
Tactical Camera
Game changing:
Capture Unique Improvements
Purely Cosmetic:
Pedro Hair fix
Other:
Take Your Time Ultimate
This last one is hard to categorise, as it does change the game significantly, but, as all it is doing at it's core is making the game last closer to the length it is advertised at... then it isn't game changing per say. ie. My marathon games should come close to 1500 turns most games. But I would say that without any slowdown mod they're all over by around 800-900 turns. The only times I've ever cracked the 1000 is with
Take Your Time on.