It has to be able to stand the test of time, and speak to anime fans for generation to come....in order to do that it must
-Have lovable, and well developed characters. If people can not get attached to what they are watching then there is no reason for it to last a long time
-It must have a well developed and entertaining plot. This does not mean one that is deep, while those can make an anime become a classic it's not the end, in the end it must simply just entertain you.
-Classic themes that show up in many anime, these are harder to define, but if you notice the classics seem to have some connection to one another, even in very subtle and small ways.
I am sure there is even more then that that must be there too...but to me that is some of the best examples.