Mostly cosmetic and UI mods, a couple of gameplay-related ones. The cosmetic ones I use are:
More Lenses - extremely helpful and a feature that should have been in the base game. You get lenses for archaeologists, builders, city overlap (adjustable, default is six tiles and is very useful for districts)
Better Report Screen - I was actually using Unit Report Screen until recently, but this one is immensely superior
Better Trade Screen - being able to sort with a click to the desired resource is a great convience
Better Espionage Screen - similar to Better Trade Screen but for spies; you want to look for great works, you just click and filter where you can
Tomatekh's Historical Religions - you not only have a bigger choice of religions (purely cosmetic), but the mod also puts certain religions as priority for AI civs, so you have Victoria as Anglican, for instance
Better Civilization Icons - purely cosmetic, more culturally appropriate icons
Unique District Icons - also cosmetic, the unique districts are easier to see
Real Great People - all great people have a portrait and the Great Prophet section is moved to the far right, so when these are exhausted there is no irritating empty section in the middle
Historicity ++ - expanded historical descriptions in the Civilopedia
Ethnic Units for City-States - basically the same thing that AI civs get, applied to city states
Also a couple of gameplay mods, though minor ones (I hope):
Less Warmonger Penalty - doesn't seem to help much, but still
Diverse Beliefs - add a few more beliefs, some may be somewhat overpowered, but at least AI civs have access to them as well
Gold Resource - I just like it from the Australian Outback scenario
More Barbarian EXP - I join those who believe that barbarians offer too little experience in the vanilla game
I have also Yet (not) Another Maps Pack. Apart from the very large maps, it also offers some very interesting map scripts, which allows for some very interesting map scenarios.