Who or what is tougher than a Jedi/Sith? Or for better, who or what could defeat a Jedi/Sith?

The ring is tougher. In the simpsons/lord of the rings crossover comic, it turns out that a lightsaber, force and even SUPERMAN can't break it.

Well, in theory, many things COULD defeat a Jedi or a Sith, such as a bounty hunter, a dark Jedi, etc. This just depends on how the situation runs its course. If you read the books, you will find that many Jedi or Sith (particularly the Sith) will lose their head, get arrogant, and therefore are defeated by something that is far less powerful than they are. There are also creatures that are oblivious to the Force and cannot be tracked by the Force, such as the cthons and the taozin from the book Darth Maul: Shadow Hunter, and these creatures could easily defeat a Jedi or Sith in their natural environment, if it is played out in their favor. Hope this helped :)

Well, i would say someone in the presence of the Ysalamiri, a creature that repels/negates the force.. but like andrew said.. it all depends on how things are played out.. a jedi/sith is still amazing even without the force, but the Ysalamiri DOES create a big disadvantage. :)