think the most frequently visited one in the bay area is probably the steinhardt, but thats not really worth making a trip for, unless you wanted to hit some of the other attractions.
as for monterey, i have been a member of MBA for 7 years, even though i now live a little too far for a day trip to visit. also i had the priveledge of doing some research dive work with a few of their researchers.
if you are looking for tropical fish, this isnt the place for you. But they have a 1 million gallon (yes 1,000,000) exhibit with sea turtles, tuna, and bonito (only other one like it is in japan, in fact, it is the largest glass window in the world at 54 feet wide, and the tallest aquarium in the world), a kelp forest exhibit of 125,000 gallons, as well as hundreds of other fascinating displays. Forget the stats on the shark tank, well over 75,000 though. The focus of MBA is the coastal regions off of california, so they have a very extensive collection of invertibrates and fishes from the area. There is an amazing jellyfish exhibit, if those intrest you. There is a section filled with shore birds, demonstrating the transition from sea to land, there is a demonstration of a river habitat with salmon, and who can forget sea otter exhibit? also there is a deep sea exhibit that is unlike any other in the world to my knowledge.
admission is like 17 or 18 dollars for adults, less for children. if you have a student id, its even cheaper.
links not working from swf at the moment, but you can visit the site at
www.montereybayaquarium.com
have a nice vacation!