I enjoy both of the main ones because you can take as much or as little time as you want. The gorillas and tigers aren't always out so some trips are more enjoyable than others.
The trails around the Tree of Life are also very enjoyable, not only for the animals but the beautiful birds as well. Rafiki's is last on my list as it seems nothing more than a petting zoo. And if the train breaks down, you're stuck there until they get it repaired. My niece and I were stuck in a long line and waited probably close to an hour to get back to civilization!