The worst is kindly vague. Many presidents were only elected one term. That puts Obama ahead of Jimmy Carter and George H Bush. Some presidents were so bad that they were assassinated. That puts Obama ahead of 4 presidents in that respect. Two presidents were impeached for inappropriate behavior. That still keeps Obama away from the bottom. One president resigned while in office. Obama doesn't strike me as a quitter. Again, I can't put him in last place. The likelihood of the worst president being the current president is highly unlikely.
I would need some more insight to determine Obama as the worst president. I would also need at least 5 runners up just to make it a fair analysis.
Obama has certainly not been a great President.
But when forced to choose between good or not good, I'd have to choose good.
If Obama had been white instead of half white, and all the fanatics had not opposed him at every turn, it is possible he might of been a great President.
But we'll never know.
In any event, most of his failings are due to his opposition, rather than his individual merit.