I have recently watched some interesting Sims 4 design videos that made me inspired to buy another expansion pack (as they are 50% off). I am considering buying theses expansion/game packs:
Tiny Living
Cool kitchen stuff
Dine out
Seasons
Discover university (I would only be able to play on one of my computers)
Get together
Island living
Toddler stuff
&
Eco lifestyle
So... There's a lot to choose from.
@elulu8 No packs released after October 2019 work with the Legacy Edition. So that includes not only Discover University but Tiny Living, Eco Lifestyle, Nifty Knitting, Snowy Escape, and the Star Wars pack.
As to your original question, I'll defer to people who actually have some kind of building skill; that doesn't include me.
@elulu8 Of the ones you mentioned, and since you cannot use Discover University or later packs with Legacy (as puzzlezaddict mentioned), Seasons 100% hands down. I see the 'Seasons' pack as the most basic/core pack to add, if I were to only add one pack. Seasons adds lots of features, plus it adds the gardening careers > Botanist career and the Flower-Arranging career. I've used flower-arranging as a nice combination for my retail stores in 'Get To Work'. And I love all of the gardening aspects of this pack! If you don't have 'Get to Work', you can still work in the career and build the skill and sell arrangements - which can be pricy with certain high quality flowers from your garden. I've had a lot of fun with the features of this pack... and it is WAY cool to see it snow and cover their world... There are a lot of game-challenging parts that are a cool switch-up in your worlds in this pack. I see Seasons as a near-essential enhancement to all of the worlds. It adds challenges (weather, etc.), realism, holidays, and so much more!
Get To Work is my 2nd favorite pack. It allows you to build, purchase and run retail stores, and I use it a lot. The stores (retail) are my favorite if used to sell my sims' wares. You get a pretty nice profit if you sell high quality skilled items your Sims make (or grow). My Sim households who do this as a large household working and running the store as a group are millionaires now. They do work hard and rotate managing/cooking for the store, etc., but it's a fun, great challenge. Just scale back to make it less work. Still... potential for great money! You can also join your scientist or doctor at work, work with aliens, visit Sixam, and abductions are also introduced.
My 3rd favorite pack is Cats & Dogs. The dogs I have are super cute and add personality and companionship to your household. They love swimming in the pool, too! If you get a cat, get the letter that cleans itself... the top of the line one. Well worth it! A maid & butler will help with the poo in the yard. It is pretty cool, though, if you love pets &/or cuteness. That pack also introduces the Vet career.
Sulani (Island Living) is gorgeous, and who wouldn't want to have the option to build a lovely home on the ocean, see your Sims swim, play, and interact with the ocean, and have mermaids in your game?!
I love the butler option brought with the 'Vintage Glamor' pack. I was also very surprised to find I enjoyed the Laundry Day pack for realism and for the cool country/'boho' style items it adds. That and Parenthood have some of my all-time fave build items.
I could go on and on... sorry. I own all packs and love Sims! 🙌
