The term "greatest" is a little bit vague, here; "greatest" is often taken to mean "most significant" or "most accomplished," in which case I'm afraid Dr. Paul would not even be in the running at this point, though I do expect his impact on history will eventually pan out to have been far greater over time than was immediately apparent.
I do, however, consider Ron Paul the best political figure in the history of the United States; I believe he possesses the strongest combination of philosophical soundness, consistency, integrity, perserverance, and sheer moral strength in the face of adversity ever seen in an elected politician, at least at the federal level. There are other political figures whom I admire immensely, but none who have the kind of spotless record Ron Paul has amassed.
Thomas Jefferson was a greater genius and more significant and accomplished figure than Ron Paul, but he did not have quite the philosophical consistency Dr. Paul does, sometimes bent under pressure, and had significant personal failings (see his continued slave ownership) which detract from his legacy. I do believe that Grover Cleveland had integrity on a par with that of Ron Paul himself, but was not quite as philosophically libertarian and did fail to veto the Interstate Commerce Act, allow the continuation of discriminatory policies against Asian immigration, and voice opposition to women's suffrage after his presidency- and in terms of personal scandal (something of which Ron Paul has been exceptionally clean) he did have an illegitimate child with a woman of ill-repute. Robert Taft did wonderful work for many years in the Senate, but he has some significant blotches on his record, as with his support for the institution of the disastrous Social Security system. Barry Goldwater was rather too warlike for my tastes. Even Rand Paul did a bit of lying and backpedaling on his principles in order to win his election to the Senate.