It was fun making this list. In my mind, I tend to appreciate the later packs more, but when making the ranking I realised I'm using so many pieces from the older packs.
10. Wetlands - I know it's a popular pack and I love some of the animals (especially the crane, capybara and ASCO, while the caiman also grew on me), but for some reason it's not the best to me. I guess I prefer a pack that feels more connected, with animals being from the same region, as this allows you to build a section based on one DLC. I also prefer the packs with scenery, especially since I'm unlikely to use a good number of the animals from this pack. Like I said, there are some amazing animals which complement previous DLCs, but others feel less useful to me.
9. Arctic - I think this one is not the best, but still underrated. The polar bear and reindeer are needed animals. The wolf and sheep aren't. The scenery pack wasn't necessary, but still contains many items which remain useful till this day, especially considering the fact that we still haven't gotten a better wood construction set after over two years. Even the decoration wooden items are surprisingly versatile and sometimes help fill the gap of a missing Southeast Asian DLC.
8. Australia - There are some nice themed items, but in general the theme doesn't make too much sense. Add to that the fact that they added the dingo, and it's easily one of my less favourite packs. Still, it contains some of my favourite scenery pieces including the fences, planks and the rusty metal items as well as much needed representation for Aboriginal culture.
7. Southeast Asia - I used to rate this pack higher, as I love Southeast Asian species. It's also the pack that fits together best, as most species somehow share a natural habitat. Hence, it's perhaps the only pack that really allows you to build a full, coherent zoo section just using animals from that pack. However, choices were made regarding this DLC which were not great. As happy as I am that Southeast Asia is now the best represented region animal-wise, I still miss a good theme to represent it. Some very characteristic species were left out (granted, we got a gibbon and the ASCO eventually), which gave place to less demanded but nevertheless very useful animals. Looking back, I am not sad now that we got a dhole and binturong instead of a gibbon and ASCO, but still it's arguably one of the least diverse packs (7 mammals, 4 carnivorans) that should have not been the first animal DLC.
6. North America - I actually like this DLC a lot, but haven't gotten to use it yet as most of the animals are from colder regions. Purely objectively speaking, however, this is probably the best animal pack so far. It has some of the missing large animals, high species diversity (3 carnivorans incl. one pinniped, one ungulate, our first 2 rodents and one reptile), most species are highly wanted and also have unique behaviours or are unique in other ways: we got three diving species, two that dig, and one that builds dams with those that aren't part of these categories being the very much wanted moose and cougar.
5. Africa - An amazing DLC in many ways, but one that seems to be scrapped together from different ideas. There's three South African animals and then the fennec fox and a North African theme. All animals are amazing and the theme is great, especially the mosaic tiles. What's lacking is a new construction set, 4x4m mosaic pieces and coherence between the whole thing. While it's one of the best packs by content, it would've worked better to get an African animal pack with the rhino, penguin, meerkat and other species and a Middle East/Arabian scenery pack with the fennec and some other animals.
4. Europe - I think the animals are fine, though I'm unlikely to use anything but the fallow deer, which is probably the best model the developers have ever made. The objects are generally too themed, but there are some very versatile ones as well. I'm especially really grateful for the rustic stone set and the decal pieces, both of which feel like a game changer.
3. South America - I used to rate this pack lower, for the absolutely ugly grid pieces (I believe this is the only thing Frontier has ever made for this game which I am so vocally disappointed about), but looking at the whole set, I am actually grateful for this pack. All animals are great to have in game (but why the Central American frog?) and it has many amazing non-grid pieces. It's probably the pack from which I use most scenery since a while, since I started preferring the ruin pieces over the faux rocks.
2. Conservation - In every way an amazing DLC. The animals are nice, though for me the siamang stands out by miles for its branchiation behaviour. The scenery is also good, as there are no useless pieces at all (except the statues perhaps), but then there are fewer pieces in general. While not as versatile as the Europe or African DLC, this pack definitely comes with some of the best scenery items and more foliage than any of the previous packs.
1. Aquatic - Obviously, this pack was a game changer, as it gave us diving animals. It also contains a lot of useful objects, including the faux rocks and wall papers and a flexicolour variant of the vertical plank set which I ended up barely using. It's not the pack with the most useful scenery - by that criterium I would rank it behind numbers 2-5 on this list - but the selection of animals definitely compensates for it and puts the aquatic pack higher than others.