@dragon: if you've been to all of those zoos then you are extremely fortunate as you provide an impressive list. Was it too hard to put them in order, or are they all different in their own special way? Do you have one favourite that stands out?
@NZ Jeremy: here in North America I'd have to say that the Georgia, John G. Shedd and Monterey Bay Aquariums are regarded as the unofficial big three in terms of quality and size. There are many others that are well regarded (the Seaworlds, Vancouver, National Aquarium in Baltimore, etc) but those first three seem to be consistently ranked as the best of the best. Monterey Bay is perhaps more accessible for visitors, as many people take trips up and down the California coast. That way they can hit San Diego, L.A., Monterey, Portland, Seattle, Vancouver, etc.