Been here twice now, and I would come back for a third. There are a lot to explore if you're not just interested in seeing the animals and plants, but actually take time to read the information and learn. Even after 2 visits I don't think I've been able to look at everything.
My favourite is Ray Bay - there's something mesmerizing about the sting rays (some giant and others cute and small) just peacefully gliding along in the giant tank. The music they play in the background is jolly and Disney-like, which I found a perfect complement to creatures and their demeanor. Then, you must follow the path to the upper level where you can try to touch them. After patiently waiting and numerous tries (they seem to play with you - often seems to come really close but are just far enough to be out of reach), I finally succeeded!
Other galleries worth noting are the dangerous lagoon (a moving walkway through the underwater tunnel where you'll be surrounded by sharks and other fish/sea creatures) and planet jellies (the display of the neon-colour jellyfish is encaptivating - you won't be able to help but take a silhouette picture :).
Great place to take young children for a family outing, or alternatively for a date. I've heard from others who have attended corporate events here - it'd be a great venue for it. I've also been to the aquaria in Chicago and Vancouver - I think this one in Toronto is slightly smaller than the other 2, and thus have less to see, but nevertheless still worth a visit!
Tip: if you're not coming for the show, come after 7PM and get a discount on the ticket. There is also a lot less people so it's a win-win situation!