Tough question - - Yes Biden has been horrible, but when Jimmy Carter took office in 1977 and unemployment was at a bit over 7% He went on a liberal spending spree and called on the Fed print more money which caused inflation to climb to 13.3 percent - - - add to this his complete failure in dealing with the Iran hostage crisis and at this point I would have to say its pretty close to a toss up as to who is the number one biggest failure.
of course Biden still has more time to get to a solid number one and it does seem as if he has a good shot at it
Edit - - on the bright side Jimmy Carter did open peoples eyes which led directly to Ronald Reagan - - so perhaps we might get something similar once Biden is through screwing things up - - - - I can hope