there's this room right off of the staircases in the AD white room - it's TOTALLY silent. like if you open a bag of chips, everyone stares at you. it's great for people who need silence to study.
the ad white room is a popular one, but it's small and old imho. i love mann. tons of computers and HUUUGE desks. great for people like me who like to splay out all their books and papers, etc.
Kinkeldey in Uris, I believe, is eerily quiet. A good thing, but you feel like an a-hole when you make the slightest sound (dropping a pen, etc.)
My biggest problem with Cornell libraries is that many parts of the older ones are not laptop friendly. This is why I love Mann and Cox-outlet at every desk.
Many people socialize in the downstairs atrium part of mann, but if you need absolute silence go to the large study desks on the 3rd and 4th floor. They are excellent, and have lovely views. If you can put up with some white noise of people talking all around you, the first floor is good (between classes on the ag quad, when you have a short break and don't want to go upstairs).
Cox in Lincoln Hall is awesome. A light and outlet at every desk, complete with a fancy desk chair.
Cocktail lounge is my place for staying up all night writing a paper. Harry Potter library is sweet for reading Plato and Dickens and feeling smart.
Screw the fishbowl.
Periodical room and asia/map room in olin.
The fine arts library is small, but the layout is interesting. I go here for philosophy papers, because I find the design inspiring (high ceiling, etc)
Africana is good if you're a lazy freshman and want to stay on north-it's small though.
I don't like the Hotel library because I feel like it has no character (though the same can be said about Cox, but Cox is more comfy).
The other libraries I feel aren't worth mentioning. Carpenter if you're an engineer, I guess. The only ones I've never been to are ILR and Vet
I haven't been to ALL 50+! 
lol, JK.
