My must have main pack is, of course, Vampires. That pack entirely changed my game for the better and anyone would have to pry it from my cold, dead hands after a battle to the death if they wanted to take it away.
Other packs that I currently can't play without are Werewolves, Realm of Magic, Jungle Adventure, Journey to Batuu, Paranormal and Spooky Stuff.
Some packs that I "might" be able to play without, but would make me miserable to do so, would be Strangerville, Island Living, Horse Ranch, Cats and Dogs, Get to Work (Aliens!!), Get Famous (for the costumes alone... I NEED them!), Get Together (Windenburg is too awesome and the club system!), Vintage Glamour (I love Butlers!), Romantic Garden (Wishing Well), The Goth Kit, Castle Kit, and Fashion Street Kit. Crap. And City Living. I love using the city so much. Maybe Seasons too. I love thunderstorms.
Oh, who am I fooling, I need them all!
For mods though, my one and true mod that I can't play without, is Bienchen's NoAutonomousPhone mod. It has completely altered the behavior of my Sims and given me peace. ❤️
I also love MC Command, specifically for the risky vampirism and woohoo adjustments, and a few other conveniences. It adds some unpredictability that I crave. Mods really can tweak the gameplay into customizable perfection. It's just really hard to keep them updated with every new patch so I keep my mod stash limited and manageable... for now.