If by 'must see' you're meaning great quality rather than inane PoSs cartoon network airs for kids after grade school I may have something to add.
Neon Genesis Evangelion: A contradictory and flawed masterpiece. I wouldn't necessarily recommend the movie because it really doesn't add much to the story and you're probably left feeling confused and depressed after you watch it.
FLCL: The words really can't describe this one.
Haibane Renmei: Simply beautiful.
Aumanga Daioh: A comedy that doesn't rely on crude sexual innuendo? Surely you jest.
Girls Bravo: Okay, you got me there, this series isn't so great but teaches an important lesson: if you only have fan service to offer you might as well take it as far over the top as possible.
Elfen Lied: Must see so you can spot this kind of pretentious bull in the future more easily.
Great Teacher Onizuka: Not really my cup of tea because I really didn't feel like finishing it but some people swear its the best thing ever.
On the other hand you might be perfectly happy with such garbage as Naruto or DB (Bleach is quite okay) if the only thing you're interested in is that its made in Japan (superior). Nevertheless if you're voluntarily willing to become a social pariah at least make sure its worth it.