If we're talking about animals that can be released into the wild, I'd go for Caribbean monk seal cause I really really really want them around. The Texas coast isn't much for whale watching (we've got plenty of bottlenose dolphins, but it's hard to find anything else on your average tour) but some seals would make up for it a little.
For the same reason, I could also go for the Carolina parakeet.
The Steller's sea cow is tempting as well, though. I mean, a 30-foot long manatee-like creature? Plus we know so little about them, they were driven to extinction, what, 27 years after discovery? They'd make for an amazing aquarium exhibit, and if a wild population could be established, all the better. If given the opportunity, my priority would be species that have been driven to extinction by humans.
Now, if I were to go for a long-extinct species, the Irish elk. Wouldn't that be a spectacular sight? My roommate would be on board, she's a big Lord of the Rings fan and she was excited to hear that the elk the elf king rode resembled a real animal. If I want to go back even further, indricotherium. You'd need a big chunk of land to exhibit it in, but I imagine it would attract a lot of visitors.