Must haves:
Seasons: I just can't play the sims without seasons. It gives a sense of time progression, especially now with TS4 and its calendar I LOVE the calendar and all the holidays you can make. Also I just love weather. Seasons need to just come in the base game already. What's a life sim without seasons?
Get Together: This is actually a pretty solid pack. I use cafes all the time, the club system is really useful and can be used in different ways. I use the DJ booth in all my nightclubs. I also use the bar activities (darts, foosball) all the time too. And the dancing skill is nice since it helps add some personality to the sims. I love making my sims bad dancers on purpose haha. I also use the walk-in closets on like every home lot I build since it functions as both a dresser and something that kids can play in (which has a cute animation).
Cats and Dogs: I'd like it to be more ranging than just "cats and dogs" but I need pets in my game. I do like the addition of the vet but I wish there were less sick animals walking around Brindleton... But speaking of Brindleton is probably my favorite world it's just so pretty and I love using the boats by the pier to get to the lighthouse. Also I like the little rabbit hole adventures you can go on with your dogs the lighthouse route is my favorite.
University: I also just need university in my game. Aside from being a pretty normal part of regular life, I also like the stories I can make for my sims in university housing/dorms (even sororities/frats if you use the club system from GT). It also comes with some good CAS items.
City Living: I actually really like city living... San Myshuno is probably my second favorite world. I love all of the apartments and the penthouses. The districts are done well and I actually really like the festivals even though I don't attend them much anymore... Also I use the karaoke venue a lot. The food stalls are nice too.
Parenthood: I like family gameplay, or more like I like having well-rounded life stages that are interesting to play through. While the kids and teens could still use some depth, parenthood does help a lot. I like being able to set different curfews between houses or no curfew at all (either based on parent strict-ness or age of sims so like kids have an earlier curfew than teens). The new interactions between family members. Being able to scold in different ways. Grounding. School projects! The phases can be funny sometimes and I like that there are different phases for kids and teens. And all the gameplay tied into the character values system. Character values can really make your older (YA-elder) sims have some individuality. Good or bad manners, responsible or irresponsible, mediator or argumentative, compassionate or insensitive, and controlled or uncontrolled emotions. I love the character value traits so much. They can also really help make some mean and nasty sims since the game seems to be so against mean sims... Plus they just impact how sims act autonomously so FINALLY sims will just on their own behave differently than other sims.
Dine Out: I really like the restaurants....when they work... I actually don't have such a bad time as some other players do, but sometimes food takes forever to get to the table. But I like having a range of restaurants from family diners to romantic lounges. I also like having sims go on dates there (they get a special moodlet for it). The possibility of getting food poisoning is kinda funny, and I think you get your bill refunded or something if you get sick? Still upset this pack didn't come with table proposals though!
Laundry Day: Don't look at me.... I just need more things for my sims to actually Do and laundry is a pretty basic aspect of life. I use the pack way more than I was initially expecting. I have washer/dryers in all my homes now. I've even seen people make laundromats which is a smart idea. It also comes with nice CAS and B/B.
Movie Hangout: So many of these packs just feel like they should've been in base game but welp here we are.. Movies are a nice 5+ group activity. The popcorn machine is nice. And I like making movie theaters out of bars since I don't use bars all that much.
Backyard/Patio stuff: Oh jeez this list just made me realize these are two separate packs?? Why?? Anyway, I use the patio stuff all the time (especially those plant wall/divider things). And the slip n slides from backyard stuff are a good 5+ sim activity in the summer time. Plus I love the windchimes....they sound so nice... And the bird feeder is cute.
Spooky Stuff: We got this pack before seasons came out (and seasons just recolored a bunch from it ?) but it still has useful stuff for me. The costume party, the costumes, and the pumpkin carving! I also like that if you put the candy bowl outside trick or treaters will automatically take candy from it. Halloween is my favorite holiday so this is definitely a must have for me.
Out of all of these pack though, I'd only buy Seasons, Get Together, University, and Parenthood at full price... Not many packs are worth full price to me this sims generation.