I'm sure I can qualify to be classed as a builder, I've built a zoo or two over the years.
Top 5 species:
Jaguar - South American tropical areas are always fun to build. Incorporating deep water diving and a tropical jungle feel is difficult but greatly rewarding
Both gibbon species - Island gibbon habitats with natural climbing enrichment is another favourite of mine
Mixed African savanna species - Not one single species but building a large mixed savanna habitat is always great fun. Typically includes thomsons gazelle, giraffe, white Rhino, ostrich, zebra among others.
Dall Sheep - Usually overshadowed by the alpine Ibex, i love creating a rocky taiga habitat for Dall Sheep that the sheep can actually climb around. Including snowy areas to mimic a diverse environment is also another bonus.
African penguin - Always great fun building for one of the more common penguin species, typically involves making a rocky beach habitat with underwater viewing to see the penguins zoom around underwater.
Top 10 Building Pieces:
Conservation Wooden Slats - My favourite Wooden piece for every need of flat and smaller wooden pieces. Very useful for decorations on buildings, habitat guest shelters and of course fences. Of course the benefit of 3 different sizes and flexicolor options make it one of my favourites.
Plaster Wall Panels - Used literally everywhere in my zoos, from decorative paths, guest and staff buildings, habitat shelters and more. Flexicolor and a neutral pattern allows this to be a very versatile building piece.
Metal Rods/Beams - From the Australia pack is a great sem of metal Beams and Rods. Useful for everything metal based in your zoo.
Mulch Pieces - Technically a nature item but mulch panels are one of the key items to make your zoo look real. Place some mulch in a gap in your gaps or scenery area next to a habitat and place natural Foliage over it for the best result.
Breeze Block Trim - This simple trim piece is the best piece to bring your paths to life, place the breeze block trim at the very edge of you path to give it a realistic looking path kerb. I always disable the default kerbs as they are pretty ugly not to mentioning they are prone to breaking when using elevated paths.
Wooden Arbor Beams - A very underrated building piece, I rarely see anyone use these Beams. Typically seen either on habitat shelter buildings for animals or sheltered guest viewing areas.
Stained Wood Walls - The Holy grail of wooden walls.
Aquatic Faux Rocks - Want realistic rock panels for anywhere in the zoo? These faux rocks vastly improve the natural look of your habitats and guest areas around the zoo. The small pebble piles can be sunk into the ground for a realistic rocky environment.
Primitive Rectangles - A very simple piece but a very effective roofing material for flat roof buildings. Animal shelters, industrial style staff buildings and more require the need for a large flat and flexicolor piece. Pair with air conditioning Pieces for the best roof.
Plaster/limestone Pillars - Turn these horizontally, and place around the edges of water pools in your habitat. This creates the exact material that real zoos use for their water pools without the need to create an entire concrete base for it.
Top 3 zoos for inspiration (looking at North america):
Woodland Park Zoo - The modern entrance for the woodland park zoo has been seen and recreated many times, I'm not sure why it's so popular with builders. The zoo itself has a great collection of animals located in very naturalistic habitats, one of thr greatest challenges of a zoo is creating multiple zoo areas that feel right within thr temperate biome, the african savanna with lots of green grass works rather well. Also one of the best sets of jaguar and gorilla habitats can be found at the zoo.
Omaha Zoo - Known for its indoor domes with desert or tropical animals, the zoo features a diverse collection in generally large habitats. The expansive African area is a great source of inspiration for large, mixed species habitats - I'm sure the guests will complain about the view though!
San Diego Zoo - I both agree and disagree with the source of san diego zoo as inspiration. But the sheer amount of habitats and choice of animals its hard to argue that san diego is not a good source. There is few outstanding habitat designs but using inspiration can build off what the real zoo uses. Habitats in the Explorers basecamp, lost forest and Africa Rocks are full of modern and/or naturalistic designs that help one explore how to build a good habitat.
American alligator/spectacled caiman - these can be interchangeable, depending on the theme. Both have similar needs in-game and similar habitat styles that are very fun. I love to use the semiaquatic trees, like the mangrove and the bald cypress. All in all, these are very fun
Southern cassowary - a fun ratite, in my opinion. I love to alternatively make lush enclosures or semi-open habitats. But that's just me
Black-and-white ruffed lemur - my favorite in-game primate. These are very fun to build for, in my opinion, and I love the sounds they make
Maned wolf/giant anteater - also a tie, these 2 are interchangeable with each other, both needing similar enclosure styles and/or can be combined into 1
Bengal tiger - can be in a nice tropical-savannah fusion habitat with a nice pool of water and ancient ruins to dot the area
Best Building Pieces
Faux rocks - flexicolor and can be used in almost any type of habitat
Best Zoos
Australia Zoo - the Crocodile Hunter's zoo, this is great to do a nice Aussie theme
Houston Zoo - maybe biased, but the H-Town Zoo is great because of the massive changes they're doing. See the new Galapagos Islands exhibit for details or check out the Texas Wetlands section or the South American Pantanal
Otters - always fun to build a river enclosure, so many different options of ways you can design for them and their compact size makes for more traveller terrain.
Okapi - I love the challenge of making a private enclosure still visible, dense traversable forests are hard to achieve, I love making forests enclosures
Crocodiles - I love making traversible enclosures for them, reptiles are often the first animal I build enclosure for in my zoos.
Lemurs - all round fun to build climbing enclosures for
Cats - I love the complex backstage designs for them, and their varied terrains are fun to create.
Best building pieces
In order
Plaster pieces - so many pieces that work for everything (the Africa player at was a god send)
Shiny Metal pieces - I use them in everything, I really wish there were more sizes and styles like the plaster set
Mesh pieces - same as above
Glass pieces - these are a blessing for making any size window design
Best zoos
1. Australian capital territory zoo - for floor plan and enclosure design
2. The unzoo in Tasmania - most realistic and true to habitat enclosures for Tasmanian devils.
3. Lone pine koala sanctuary - amazing reptile enclosures
(Sadly I have only been to Australian zoos so my opinions are skewed)
Californian sealion specifically I just think the grey seal is boring
items I use most
east asian wood pillars
arctic wood set I love it as a trim
plaster set
mesh pieces
the metal fence post that looks like a hinge
australian wood set
shingle roof pieces
dry stone set
modern glass pieces
new world fence pieces
zoos I have been to and liked since I generally dont use zoos fo inspo when building
taronga zoo since it is my local major zoo though honestly the building are sort of meh
sydney wildlife zoo probably not good for general recommendations since it has a very weird mostly indoor layout but I love it for small exhibit designs and how to make good use of space.
featherdale wild life park a very australian themed zoo I believe all or at least almost all the animals kept are australian natives great example of the diversity of oceanic life.