I have been to both Monterey Bay and Vancouver Aquarium, and they are two of the better institutions around. Of the approximately 50 aquariums in North America, they are both in the top 5 in terms of size. The Vancouver Aquarium is currently around 2.1 acres, and is adding 1.5 acres within the next 2-3 years. They are expanding/adding many outdoor pools for their dolphins, sea lions, harbour seals, sea otters and beluga whales. Plans are to greatly increase the size of the habitats, and perhaps add newer additions such as puffins and beavers.
Monterey Bay is relatively new (1986?) and is a fantastic place to visit. I went there last year and was impressed by the hands-on exhibits for kids, the concave glass on select tanks that curves inward rather than outward, the enormous kelp forest tank that is 3-stories high, and the 1 million gallon tank that holds some of the largest fish imaginable.
Vancouver Aquarium is a place that I've visited at least 7 times, and plan to go again within the next few months. They have larger animals there, while Monterey only has sea otters and not any dolphins, seals, sea lions or beluga whales. Vancouver has a tropical section that is stellar, with visitors beginning to literally sweat after a minimum of 20-30 minutes wandering around this "amazonian" exhibit. There are a few marmosets, two-toed sloths and bats alongside this section of the aquarium.
I haven't been to either Shedd or Georgia, but it is interesting to note that Georgia claims to be the largest on the planet. There was nothing in Atlanta a few years ago, and then one of the founders of Home Depot Hardware Stores splashed out a massive amount of cash to build the world's largest aquarium. Voila! Sales have been through the roof, and the whale sharks and beluga whales are the top draw there.