I certainly hope Nixon was the worst, but I wouldn't be surprised. Clinton sleeping with the intern girl--wow, THAT's a shocker (nottt!). (Why it's anyone's business but Hillary's is a shock, though. Just destracted him from running the government.) Hoover was no prince, persecuting innocent people on trumped-up charges. Kennedy was a pretty-boy (who slept with more than ONE person other than his wife on a regular basis, but we forget that), but mostly ineffective. Johnson actually got it in gear and completed most of Kennedy's projects. Carter's head was in a noose his entire presidency, so it's not surprising that the Iranian hostages weren't freed until the day after Reagan was sworn into office--just to spite him. Carter had to be the most honest president next to Abe Lincoln, at least, because he was flat broke when he left office. Obama is plagued by a highly polarized congress and nation, under a too-fine microscope at all times, yet we blame him for not accomplishing "anything" (what president could with this congress?), though Obamacare is a godsend to the people who need it as are many of his other successes, hence his Nobel Peace prize.
Worst presidential crook EVER? Not sure if it was Nixon, but in recent history (Nixon and beyond): yes, he's probably the worst crook or we'd have started hearing about it through the unofficial, shameless biographers of the world who are out to make a buck at someone else's expense, denying anything positive they may have done while in office. Bottom feeders can find something wrong and blow it way out of proportion for no reason if they want to, and the bigger they are the harder they fall. (Have I used enough cliches?)