Gary''s Student wrote...
....
Function superfoo(r As Range) As String
Dim s As String
m = Application.Max(r)
superfoo = r.Find(What:=m, After:=ActiveCell).Address
End Function

This throws a #VALUE! error.

What am I doing wrong?
....

Find is a *method* rather than a property of the Range class. You can't
run methods from udfs because udfs can't do anything other than return
values to the calling cell. The Find method tries to change ActiveCell,
but udfs can't do that.