If it's replacing a subject, then 'I' is correct:
You should do it, not I.
However, in common usage, 'me' is more commonly used. It depends on whether you want to be pedantic or not.
To be really, really, very pedantic, "me" is correct, not "I".
The idea that "not I" is correct is a basic fallacy caused by a grammarian who thought that the difference between English "I" and "me" is the same as that between Latin "ego" and "me" -- but it isn't.
However, this mistake was so often copied that its use became widespread, but not all grammar books agreed with this usage, and it was never standard English. However, because the error was made by the well-educated, it was assumed that it was correct formal English.