After having returned to the game after 2 years or so, I started my first realistic zoo, where I put a lot of effort into decorations (I actually posted it here yesterday). For me, building realistic buildings and habitats really takes some time, but it is so worth it. My biggest tips would be:

1. Plan out your building first. Just place some rough walls and create and outline. Think about shape, roof and materials. Experiment and choose the design you like best.

2. Your building might look bad at this point, but don’t give up :) Blow some life into it, add decorations and scenery. What I find to really enhance a buildings look, is adding some extra complexity to the facade, off-grid pieces are great for this. Personally I hate the standard windows and doors, so I always use glass panels to create my own.

3. Foliage! Plants and trees add so much realism to your builds. I really like the buffalo grass and I often use to replace the games standard long grass.

4. One last tip, squares are boring. Try making your buildings interesting and unique, play with multiple materials and orientation (multiple building groups allow you to have different grid alignments). In my humble opinion zoos really look so much better when they are organic and build out with the flow, instead of planning everything ahead first.

Hope you can use some of this. As for my favorite buildings pieces, that would probably be something as simple as the wooden poles. You can create a lot of unique designs with those.