Okay, this reply ended up being waaaaaayyyyy longer than I expected. But I hope it helps?
It really depends on what kind of gameplay you'd be interested in to what you should get.
Game Packs
If you're into supernatural things and want to play with/as a vampire or witch then I recommend either getting Vampires or Realm of Magic for your game pack. In both Vampires and Realm of Magic, you start at the bottom ranking and become more powerful as you move up into the ranks and gain perk points which can be spent on powers (vampires) or spellcaster perks (Realm of Magic) Because they have their own game packs dedicated to them. they will allow you to have a very fleshed out experience with either of them.
But if you're more into family play, and like the idea of raising kids and starting a family, then I would go with Parenthood. I'm not much of a family player myself so Parenthood is one of my least favorite packs, but those who do play a lot with families really seem to love it, so I'll recommend it anyway.
I would stay away from Dine Out, Batuu, Dream Home Decorator, Spa Day and Outdoor Retreat for now. Dine Out because it's very bugged and it takes forever for your Sims to actually get their meals. Spa Day and Outdoor Retreat, because even though Spa Day did get a refresh and is a lot better than it was previously, a lot of us tend to feel both of them fall flat compared to other packs. I still think they're worth getting, but at a later time. I wouldn't recommend them as a beginner pack. DHD doesn't really offer much in the gameplay department and is more of a glorified stuff pack if you ask me. It's really more of a pack for builders. Batuu, well... it's a very controversial pack to say the least. A lot of people feel it's just a big old cash-grabbing advertisement for EA's Star Wars games and I've heard that due to limitations of what can/can't be done, the gameplay is lacking. I don't have it though so I can't say for certainty.
Strangerville and Jungle Adventure, although I haven't fully played with them yet also are said to be really cool packs, but there's really not a whole lot of replay with Strangerville since once you solve the mystery, you've solved it. And Jungle Adventure is more replayable than Strangerville from what I heard, but sort of falls in the same category of "you only want to explore the ruins so many times. Again, I'd wait on these if I were you.
Expansion Packs
As far as expansion packs go, it again sort of depends on what you want to prioritize. Most people consider Seasons, Cats & Dogs, Cottage Living or City Living to be their favorites.
Cats & Dogs and Cottage Living both come with animals and are similar in that respect but also quite different. Cats & Dogs as the name suggests focuses more on the pet aspect being centered around, well... Cats & Dogs. And Cottage Living is basically the farming expansion pack and comes with more farm based animals (cows, chickens, llamas) and also wild rabbits and foxes. The animals in Cottage Living are not editable in CAS and you can't add rabbits or foxes into your household, so this one falls behind Cats & Dogs for me. Cats & Dogs although it comes with only cats and dogs I think has much better, fleshed out animals. You can create them right through CAS or you can adopt strays you find on the street after befriending them. They each have their own unique personalities and quirks too, just like real pets. Some people are disappointed that you can't actual play as the pet or control them, but to me it just makes it seem more realistic and like real life. Also in Cats & Dogs, you can own and run a Veterinarian clinic. With Cottage Living, you raise the farm animals, but also crops which you can enter into competitions at the Fenwick Fair. And you can also run errands for the townies as well. And both packs can contribute nicely to family play. But Cats & Dogs also contributes to occult play seeing as you can bind your pet as your familiar.
Seasons, like the name suggests adds Seasons, Weather, and Holidays to the game which is definitely a game changer for sure, but if you're looking for something that adds a lot of things for your Sims to do, it doesn't really have that much other than the different holiday events. You can also create your own holidays which is really cool, and the different Seasons and Weather really makes each world feel different. I do really like this pack, but I'm skeptical about recommending it as a "beginner" pack since it doesn't add a super big amount of new gameplay, but rather enhances the gameplay you already have and making it feel more true to life.
City Living, while not my favorite does add A LOT to the game and is fun. I'm just not super into city life. It not only adds apartments to the game but festivals, and flea market too as well as different careers. You also get a bunch of lot traits & lot challenges with this pack which can be pretty fun. The festivals do give your sims a something to do and you can treat it as a fun outing, but they don't really change much in regards to activities from week to week. They're definitely fun to go to multiple times of course, but I recommend splitting up festival visits as opposed to going to one every day during the week. (ie go to Geekcon in one Sims week, but save Romance Festival for another one.) I'd feel comfortable recommending this as a starter pack, definitely.
Eco Lifestyle, Get Famous, and Discovery University are all very niche expansion packs if I'm being honest. They're not bad or anything but unless you're super into the environment (Eco) fame Get Famous) or college (Uni) I would hold off on them.
Get Together, Island Living, and Snowy Escape are also really good options. Get Together has a club system which can be incredibly helpful and there are a lot of unique lots like the bluffs, ruins and the chalet gardens. Sulani is super fun not only for being able to go to the beach, but there's a lot of culture in Sulani as well. Also, mermaids and dolphins. :) Oh and there are some events that take place there as well. Snowy Escape mostly revolves winter sports like snowboarding and skiing, as well as activities like hiking and sledding. There are also some festivals in Mt. Komorebi, but not as many as San Myshuno has in City Living. Still very good packs in my opinion.
Get to Work is a pretty decent pack as well that adds a lot but being the first expansion pack, it does sort of feel lackluster in comparisons to some of the others. But it's also not bad. It's a sort of "middle ground" pack for me. The main focus is active careers which a lot of people seem to enjoy, but I'm personally kind of "meh" on that. Retail lots, while not totally bugged don't really work right since mannequins cause the game to crash. The retail lots still function if they don't have mannequins though, so a bakery for instance would work just fine. And GTW also comes with aliens. Overall, it's a nice pack but one I would wait on.
To sum up, these would be my recommendations.
Game Packs
- Vampires
- Realm of Magic
- Parenthood
Expansion Packs
- Cats & Dogs
- Cottage Living
- City Living
Also, if you plan on getting an Expansion + Game Pack at the same time, I'd just go for a full bundle since it's not that much more expensive when you tack on a stuff pack as well.