If you like supernaturals and can afford it, Vampires is a must have! In my opinion, it's probably the most well-done GP by far! Parenthood also vastly changes the gameplay in families with kids, and is similarly well-thought-out. I enjoy both of them, but I really have to recommend Vampires. The customization system and the CAS parts for vampires are beyond awesome, and you get three new vampire-related aspirations. Spa Day and Dine Out are fun and add good stuff to the game, and running a restaurant looks fun if a little frustrating without mods. Outdoor Retreat I kind of partially regret buying just because the trips got kind of boring for me, so when one of my sims rolls Outdoor Enthusiast aspiration, I'm always like, "Oh geez, now we have to go to Granite Falls again!"
But it added a lot of cool stuff and GF is gorgeous.
SPs kind of depend on personal taste, because they don't add huge amounts of gameplay, so it depends on whether you like the CAS and build/buy assets. Personally, I think Kids Room Stuff is great if you're a family or legacy player, because kids always need more stuff. They just did a teaser the other day that a toddler pack will be coming out late this summer. An SP, I think. I personally like Backyard Stuff. Also Luxury Party Stuff, but that's because I
enjoy putting my sims in ridiculous outfits. I really like the boho clothes from Movie Night. Otherwise, you'd do best to watch
LGR's reviews to see what's on offer. I don't always agree with him, but he's funny and does a great job of showing what comes in a pack.
EPs can be kind of a mixed bag. They can add a lot to the game, but some things always seem a little unfinished or haphazard? Like I love living in a penthouse in San Myshuno and customizing it, not to mention other gameplay stuff, but if you think too hard about apartments in Sims 2, the differences get kind of depressing. I wrote about my EP opinions more extensively
here in another thread, so I'm not going to rehash them.
THE must-have mod is
MC Command Center. He's trying to replicate some NRaas functionality from TS3 and add more control and a modicum of story progression to the game. Very responsible about updating. By the way, any game patches or updates have a tendency to break a lot of mods, moreso than previous games in my opinion, so always check if your mods have been updated after any patches. Packs tend to break things less, but still can, but the updates are occasionally really hard on the modding community. MTS has a lot of great mods for gameplay tweaks and annoyances. There's a lot of mods and CC there and of course on Tumblr. A good place to look for CC and mods is
Lana CC Finds. It's how I found pretty much all of my toddler CC, because the annoying thing about looking on Tumblr is all the good stuff is scattered to the four winds instead of being localized anywhere, but I digress.
I don't post here a ton anymore but I lurk, and I play Sims 4 a lot, so if there is anything you wanted to know in particular, just let me know!