I think that "great" is a difficult adjective to pair with "Republican." Nevertheless, I think that the last decent Republican president was probably Eisenhower (John Foster Dulles notwithstanding); at least he warned us about the looming military-industrial complex.
As far as a truly "great" Republican president, I'm going to have to go back to the beginning and nominate Abe Lincoln, as keeping the Union together and championing the humanity of African-Americans are pretty decent resume builders.
