Card Packs have a predetermined amount of rarities depending on the pack. Most of the standard ones carry 6 Common, 3 Uncommon, and 1 Rare iirc. There are some like the 20th Anniversary packs that have way better draw than that, but they are uncommonly sold and probably expensive. So if you're looking for which packs have the most rare (rarity) cards, only special cases like the aforementioned is better than standard packs. :P
If you're in it for the money, a lot of the newer stuff isn't selling for higher than 50 USD in very extreme situations, so you probably won't get very expensive ones out of those as of right now.
I'm not sure if Walmart is still selling pre Sun Moon card packs, but the value is really found before then. I know the XY era cards are starting to get pretty good prices, especially XY Evolutions, because of the classic card design, and the rarest of those breech 500 USD from last I checked- sometime in November of last year. Even the Standard Charizard from that pack Sells high right now. If you see anything like Steam Siege, BREAKthough, or BREAKpoint, I've heard those fair good as well. I will, however, advise against doing this often, as I've seen people buy a thousand or so of BREAKthrough and XY Evolutions packs and barely break even with their investment by selling all the Rares, making a small amount of money selling the commons and uncommons as well. And if they didn't buy the whole thousand, they probably wouldn't have gotten a high grossing rare- and would then lose a lot of money. There isn't much net gain with stuff you can find these days, and you need to buy a lot to decrease risk while also decreasing reward.
Walmart may not even sell these cards where you are, and of course cards older than XY have value to them as well- so long as they are mint/near mint.
Another bit of advice: You also have to think about how the Rare cards are separated from each other- Ever Since halfway through XY or something like that, the standard card packs have been divided into two groups- ones with Green TCGO card codes and ones with White and Green TCGO card codes- the white ones tend to have rarer or better rare cards. Unfortunately, you can't tell if a pack is one until opening, but you can have higher hopes when you open a White pack than a green pack. :P
If you wanted good rares to play the card game, however, I suggest you don't buy packs very much and just order them online if able. As with anytime you order anything online, be sure to check if the seller is reputable. I reccomend TGC Player, but there are plenty of places online that you could get them, so go with whoever you trust.
If not able, find hobby stores near you that sell individual cards slightly over (or sometimes under) priced and plan what kind of stuff you want for your deck ahead of time. If you can't do either, then you kinda out of luck; at that point, look at what each card set contains, find one with a lot of stuff you want, buy a bunch of packs for it, and hope you get enough cards you needed to slot into a deck. That and trade with someone else. There isn't much you can do after that, unless you join a draft event, where they give you cards to build a deck that you keep after participating, regardless of your placement. :P
Those are my nuggets of knowledge on the topic. Either way, good luck in your endeavors!
Source: A lot of Experience
Hope I Helped!