Personally while packs in general are mostly subjective, there are a few criteria for being exceptional or for missing the mark.
For scenery packs, to me a major factor is how good is the scenery and how much do i use it, and id say all of them are atleast good, but imo the arctic wood, south american bamboo, australias metall, aquatics godly fake rocks and wood, europes rock walls and many smaller pieces and decals, conservations everything and twillights wood and metall all are a cut above the rest, being pieces i cant imagine building without anymore. The only pack i didnt mention was africa, which has nothing to do with the quality of the pieces (its pretty great) but the kind of zoos i build, being mostly european light to medium themed zoos, while the africa pack really opend up possibilitys to build with a completly different style.
For individual animal choices, its a mix of how common it is in zoos, how different it is to actually build for them (for example the water buffallo encourages to build large pools of water into their habitats, which you defently could do for the cape buffallo or bison, but it makes a bison and a waterbuffallo habitat look very different from each other 9/10 times), how flexible the animals are (The dhole is my favorite animal not because of any big subjective reason, but for how flexible they are in their use. Giant range, from snow filled valleys over temperate forests, suptropical grassland to tropical rainforest, both good as a filler and a highlight animal, these red puppers can get so many different jobs done), how different they are from similar animals in the game if they even exist (Example Donkeys. Domestic Donkey is by far the most different in apperance and use from the current equids as a domestic, making it my most wanted, the somali looks the most distinct from the wild donkeys and lives in an area we have no equid from, being rocky deserts around somalia, and the asian donkeys are at the bottom, because they look quite similar to the prez wild horse and even partly overlapping in range) and simply how good looking i subjectivly find the animal (i prefer matchies over goodfellows, cause i think matchies looks cuter).
For the overall animal choices, besides just how much i value each of them individually its also about how similar they are to each other. Great example would be the Africa Pack vs the Twillight Pack.
I personally like the Twillight pack for what it is and would rate all the individual animals as good additions, but even if i prefer every single animal in the twillight pack over both the white rhino and the fennek fox, i feel that the africa pack had a better selection, as it gave us one large ungulate, a small carnivore, an aquatic animal thats also a bird and the meerkat, which is close to infinity in how good of an addition it was. Meanwhile the twillight pack had the wombat and our first non macropod or koala marsupial, which is great and amazing, the third fox to round out the classic 3 fox adaptation diagram, our first north american arboreal animal and another small north american carnivore. Eventhough the twillight pack technically has a more diverse roster in continents, featuring 2 from north america, 1 from oceania and mr worldwide, the fact that 3 of them are small carnivores from north america is what makes the selection worse. Without even going to grab another popular animal like the tassie or the porcupine, if the skunk would have been lets say a slow lori, the pack would have been much stronger because the current roster has 3 animals that can be copy pasted into the same enclosure +- a climbing frame. I literally build a very pretty racoon enclosure and put in skunks last second cause the sign was prettier with the addition of a skunk tail. Its not a bad thing in general that we have animals that can fit into the same enclosures, afterall this gives our zoos some individuallity and choice, as even if 3 people got the same exact zoo they could very well fill it differently (hello african antelopes and the 10+ SA monkeys i still desperatly want), but its less fun if 3 out of 4 animals in the same pack can be copy pasted into each others habitat.
But while this is a dent for the twillight pack, the high quality of the indiviual animals, scenery and he first exhibit animal ever that you can actually build for still in my opinion salvages it to a nice good middle of the road pack.
Meanwhile the only pack i genuinly think is "bad" (with bad i mean more like 5/10, while all other packs are pretty much 7+/10), would be the SEA pack.
For Multiple reasons, with the most objective one being the bad quality of the animals not in choice but how they are in the game. While the bintuwrong and the dhole have been fixed, the clouded leopard still is the worst climber in the game and the painted bairdeds tapir is exactly what it sounds like.
Then for the actual choices, its also kinda eh. We got a very hate it or love it bear, the smallest big cat, the dhole which i love but i totally understand if someone just disregards them as a red dog, the malayan tapir which would have been nice if it wasnt yk a clone, but still good animal choice, an monkey that straight up doesnt exist in capitivity outside of indonesia and japan, when we really could have and still do need a more general south east asian primate even after the siamang like the sulawesi crested macaque, East Javan Langur, red shanked douc and many more and also the pretty dope binturong and babirusa, with the binturong being like nothing else in the game and the babirusa our second suid that looks very different from the current one. And the exhibit is an old one aka a joke. Whenever i try to remember what was in the south east asia pack i feel like somethings missing, till i realise i allready counted all 8 animals and yeah something is missing. Out of 8 Animals, only 2 of them are what i would call without a doubt a good addition. Ofcourse thats also subjective, if you find that the clouded leopard is your favorite animal, im sure its a blast for you, but even when having 8 animals, id only feel like 3 of them really are worth using over other animals and add something new to the game, and mind you one of the 3 is the dhole. If we would get 1 or 2 more general asian monkeys, im sure the proboscis monkey will sting less and will be counted aswell in the future as they are quite unique as swimming monkeys that look like squidward, but for now its the only pack that if i wouldnt allready have it, idk if id care enough to buy it outside of supporting the developer.