Zoo Tycoon 2 with all of its expansions are just phenomenal. If your computer is too slow to run them, I recommend getting a new one. My computer's fairly old and runs Zoo Tycoon 2 with every expansion without any issue. My favorite has to be Marine Life, because I love the whales and dolphins. But the Dinosaur one is also pretty sweet.

Also thank you for this thread. It's nice to know that there are still some zoo tycoon lovers out there.