Key West Octopus
Of all the aquariums I've visited -- and it's a somewhat embarrassingly high number -- Key West's one-room depression-era setup has always been my very favorite. For a start, its small size means that you'll spend very little time fighting your way through hordes of other visitors. There aren't many places like this where you can sit and wait (and wait, and wait...) for a decent octopus photo op without irritating a lot of people.
As an added bonus, the aquarium layout is such that you're in near-constant danger of stumbling into the shark tank. There's just something about that that really piques the senses.
[Canon 50mm f/1.4]