I am pretty sure it was President Richard Nixon. Other President's may have said it since of course, but were not captured on tape saying it.

While Nixon said the slurs privately at the time, the quotes were taped by the White House recording system in the Oval Room (and those tapes are in the National Archives), so it is fair to say that Nixon was indeed "quoted" while in office, caught on tape referring to blacks with racial slurs like "niggers" and "jigaboos".

At one point in the Nixon tapes, when he was working on his first presidential address to Congress, during a conversation with Secretary of State Henry Kissinger about how to handle African foreign policy, the President was quoted on tape as saying "Henry, let's leave the niggers to Bill and we'll take care of the rest of the world".

EDIT - Here is a link to some audio from the Nixon tapes of Nixon talking to Donald Rumsfeld and saying "nigger" in reference to American blacks as well, though in this instance he plays it off as if he is just quoting Southerners.

Clinton used the word while giving a speech at the Congressional Black Caucus Foundation dinner in 1995.

"...organized groups, committed to destruction, based on racial or ethnic or religious or political extremism, have enormous capacity to do that destruction... you see it in more subtle ways, yes, even in America. Like when five children in an upper class suburb in this country write the hated word 'nigger' in code word in their school album"

EDIT: Found C-SPAN video of the speech if anyone is interested. Relevant quote starts at 14:30.

Followup question: has a sitting president ever used that word on the record in the presence of the press as a racial slur?

What does potus mean? Is it a short way of saying president?