You have to realize that the Fourth invented the technique. When Jiyara said it took the Fourth 3 years, most likely the Fourth was just experimenting on and off, and doing it by trial and error.

Of course nobody knows for sure but I believe:
Young 3rd > Jiraya > 4th >= Itachi >= Tsuande >= Orochimaru > Old 3rd

It seems to be hinted that the young third was the strongest and most skilled ninja of all history, they called him the professor.

Even Itachi is scared of Jiraya, so I'm assuming for now Jiraya is the strongest.