A whole host of most from LittleMsSam - There's 191 files in the folder in my Mods for that modder. They just do all kinds of little tweaks that are vital. The ones I interact with the most would be the more visitors and auto professionals - but that's only because others are more 'behind the scenes but still vital.
MCCC - this is vital for me to disable things like aging, pregnancy, attempted limits on numbers of aliens, remove situational outfits, and just like with LittleMsSam - all kinds of little tweaks that make the experience more to preference.
- Key in both of the above two is each have a huge stack of choices, and almost everyone can find something they want for their game. Even if what we want is the exact opposite of each other.
LittleMsSam does it right though - everything is an individual module so no code bloat. Things should never be bundled together unless they're related to the same purpose / use case. LittleMsSim often even divides aspects of the same thing into individual modules so you 'disable' an extra feature by just not downloading that package file - proper modern coding there.
I use a few NSFW mods that are critical for me - even though some of them seriously annoy me because I'll want one feature, and have to spend a lot of effort working to disable the other features... (like... why does someone put a 'vanilla NSFW' item together with a tweak that makes sim pregnancy work through laying eggs... good code design is to keep things modular. Super bundles of 1000 features is so pre-1990)
ShowSimInfo - this is a dead mod, last updated I think 2 years ago. But it still works unless you have Island Living (or so I've read), which I have yet to buy so I can still use this. It seems to be the only mod out there that lets you do something like search your entire game for every sim with 'trait X'. So I can quickly find things like 'all aliens', 'all spellcasters', or 'all sims with that aspiration to become an egg layer'... and edit them.
- It can also search a specific sim - as in tell you everything about your active sim or a sim you select. So it's not just a sim finder, but also sim detail extractor.
- very useful for fixing things when you let some bad CC in by accident and don't notice it soon enough, as you can also find hidden traits.
If somebody's got a more modern mod that does what that mod did, I'd love to know. Eventually I will get the Island EP, or some future EP - and it will break on me.
I use a lot of autonomous mods from Polarbearsims, that can get me in some trouble sometimes. Like an Autonomous Proposal mod so I'm often looking at a popup where some townie is asking to move in with another townie. I imagine that would drive some people nuts but it's exactly what I wanted - for my townies to start moving around and forming their own households. Once again, like LittleMsSam, many of Polarbearsims' mods are in individual mods so I can get 'this kind of autonomy that I want' and not have to struggle to disable that other one that I didn't want. Good principles there - keep it modular as you can.
I've got a huge stack of new traits from Chingyu. Some of them are great, some I had to delete because they would make a sim into a superhuman. All of them give errors for improper tuning setup, but still work...
Beyond this I have a huge stack of clothing and furniture CC, and I've used Sim4Studio to make most of the clothing random possible so my townies use it. If it looks bad when I do that I 'usually but not always' just delete it.
Lastly, Sims 4 Tray Importer is vital when you start using a lot of CC because you will invariably download something that looks horrendous in game either because it's badly made, it's badly flagged (like wrong gender or wrong age so it wrecks the mesh), or it just doesn't look right next to your other CC (like something 'realistic' when most of your CC is cartoony).
- and I use this often to find what file is causing an issue and deal with it. Just yesterday I noticed all my new townies and any new sim in CAS had no head, so I had to export one to the library and open both it and an older library entry that was good - and compare them in Tray Importer to find out that a 'batch fix' I'd done with Sims 4 Studio had ruined a 'default lashes' replacement I had... so I had to go get an 'unfixed' copy as the 'fixed' one was bad... Without Sims 4 Tray, figuring that out would have taken me hours and hopeless attempts to beg people for help on forums... Instead it was a 5 minute fix.