Lewinsky Scandal? You really think that or the recount would rank on any historians top 50 biggest bloopers? I don't. History extends well beyond 1990.
Picking Palin was a real head scratcher...I'm not sure it made that much of a difference in the end.
Off the top of my head, I would say giving a two hour inauguration speech so that fall ill and die one month into your term is the biggest blooper!