The Atlanta Aquarium is really excellent. They've put a lot of effort into designing the exhibits so you see the animals all around you instead of through a flat wall. There's a little tunnel where you can crawl under the penguin tank and stick your head into a glass bubble up among the penguins. And there's an enormous tunnel through the tank with the manta rays and whale sharks. It's the only aquarium I've been to that may be as cool as actually scuba diving.