1.
GTO - perfection in anime form, massively rewatchable and hugely entertaining.
2.
Azumanga Daioh - I've watched this more times from start to finish than any other show and still "lol" every time.
3.
Hunter x Hunter - my shounen action pick. Continually interesting from start to finish.
4.
Lupin III - so many episodes! I'll never get sick of Lupin, plus I'll go ahead and assume I can have all the movies and specials too.
5.
KareKano - my favorite romantic anime