One
site I found lists the top ten aquariums as:
Deep Sea Aquarium (Europe)
Shedd Aquarium (North America)
Two Oceans Aquarium (Africa)
London Aquarium (Europe)
Ubatuba Aquarium (South America)
Waikiki Aquarium (North America)
Genoa Aquarium (Europe)
Georgia Aquarium (North America)
Sydney Aquarium (Australia)
Singapore Aquarium (Asia)
From a June, 2005 article in USA Today about recent aquarium makeovers(in no particular order):
Adventure Aquarium
Camden, N.J.
What's new: Just about everything. The former New Jersey State Aquarium re-emerges after a $45 million overhaul that brings new management, a new building and many new exhibits.
Opened: May 25
Highlights: The all-new West African River Experience, home to hippos, crocodiles, porcupines and more than 20 species of African birds; a new 550,000-gallon shark tank with more than two dozen sharks, viewable from multiple wall windows and a 40-foot, walk-through tunnel.
Coolest critters: Button and Genny, billed as the first hippos ever at an aquarium, starring at the West African River Experience
National Aquarium
Baltimore
What's new: Animal Planet Australia: Wild Extremes
Opens: Fall 2005 (date to be announced)
Highlights: The 64,000-square-foot glass addition to the East Coast's most famous aquarium will house a re-created red rock river gorge like those found in Australia's Northern Territories. Unusual Australian animals, many never or rarely seen in the USA, include an Australian species of crocodile, lungfish, flying foxes, snakeneck turtles and various lizards, snakes and birds such as rainbow lorikeets.
Coolest critters: Johnston's freshwater crocodiles, a species never before seen in the USA
Monterey Bay Aquarium
Monterey, Calif.
What's new: Ocean's Edge, an $11 million transformation of the groundbreaking aquarium's original galleries
Opened: May 27
Highlights: A walk-through "wave crash tunnel" in which visitors experience what life is like for creatures that live in pounding surf; a new giant-octopus exhibit area; an expanded coastal wetland and sandy shore aviary; and hands-on stations such as a revamped bat ray touch pool.
Coolest critter: The sheep crab, a relative of the Japanese spider crab that grows to more than 18 inches across
Tennessee Aquarium
Chattanooga
What's new: Ocean Journey, a $30 million expansion that brings saltwater creatures to the world's largest freshwater aquarium. It's part of a $120 million overhaul of the Chattanooga Riverfront that includes expansion of the Hunter Museum of American Art, renovation of the Creative Discovery Museum and enhanced public spaces.
Opened: April 29
Highlights: A replica of the Gulf of Mexico's Flower Garden Banks National Marine Sanctuary, home to coral reefs, tropical fish, stingrays, turtles and sharks; the Butterfly Garden, with hundreds of butterflies from around the world; a "Boneless Beauties" gallery with invertebrates such as giant Japanese spider crabs and jellyfish.
Coolest critters: The giant Pacific octopus
Indianapolis Zoo
Indianapolis
What's new: The Dolphin Adventure
Opened: May 28
Highlights: Visitors can watch dolphins from what's billed as the world's first totally submerged dolphin viewing dome, which sits in the middle of the main performance pool. The underwater dome measures 30 feet in diameter and 12 feet high and is accessible via two clear acrylic "tunnels" at the bottom of the pool, offering 360-degree views of the dolphins as they swim. The attraction will be home to the Midwest's first get-in-the-water-with-the-dolphins program starting in September.
Coolest critters: Gene and Jane, specially trained retrievers (one black, one golden) that interact with the dolphins during daily shows
I am particularly interested in seeing some amazingly large Amazon exhibit! 
