I'm still experimenting with mods but so far these are the ones I'm not planning on removing any time soon (if ever).
WhimOverhaul by Bienchen83. Essential for me. Vanilla whims are a complete dud for they even stopped doing anything with them. This mod adds a tons of whims and makes them actually relevant to a sim's personality/mood. No more "buy some random object" whims.
MCCC by Deaderpool. Nraas Master Controller (+ StoryProgression) for TS3 is a must for me and this is similar to that. I like being able to tweak little things and make the game a bit more challenging and interesting.
Vampires Can Kill by Zero. I really love the whole "risk vs reward" element this brings to vampire gameplay. Especially with my main vampire who has the Insatiable Thirst weakness and who used to just Compel for Deep Drink all willy-nilly (minus family members) but now that there's an actual chance of it killing the victim, he's making more conscious decisions on his hunting habits. Of course, the mod also allows you to just play a bloodthirsty vampire who deliberately chooses the deadly options. Really liking it all.
Meaningful Stories by Roburky. Never liked how the Emotions system makes most of the sims act in the same bipolar way with not much of their core personality showing. This one makes the system feel way more organic and actually gives the emotions meaning. No more yo-yoing between different moods like crazy.
//EDIT forgot this one:
Parenhood hard mode by Kuttoe. Especially because I'm playing with long lifespan it's WAY too easy to maximize character values, in fact Manners and Responsibility tend to do that on their own without active input from me. This mod makes it so that the kids start at random values in the red. I'm liking it. I actually want some sims to be irresponsible and ill-mannered instead of them all acting like Mr/Mrs Perfect.