The biggest ones are MCCC and UI Cheats Extension. I'm not sure I would make MCCC my first mod though as it definitely had a learning curve for me. It has many options to allow you to control your game as to story progression, autonomy, and CAS.
As for enhancing gameplay, my favorites include Adeepindigo's Education Overhaul which allows virtual education at home, field trips, summer vacations, and private rabbit hole schools. The same modder also has Healthcare Redux which allows Sims to develop illnesses that can be fatal if untreated as well as other milder conditions. The serious stuff is rare enough that it may take you some time in gameplay to encounter it.
Meaningful Storiesby RoBurky makes the emotional system in game make more sense. Chemistry System by the pancake1and MizoreYukii is an "attraction system" not for romance but for any relationship which makes it easier or more difficult to gain relationships based on traits and personality. I started using it as a simpler system than Wonderful Whims which has some unwanted things for me. Lumpinou's First Impressions is a fun little mod that gives a sentiment based on a Sims first impression when they meet the first time.
Also check out mods by Bienchen and LittleMsSam. They have tons of mods that add gameplay often in very simple ways. If you are looking for Build and Buy objects that enhance gameplay, check out Ravasheen and Pandasama. If you want new aspirations, events, or careers, check out Ilkavelle and KiaraSims.
