"Who would win in a fight -- Darth Vader or Godzilla?"
We're talking about the Darth Vader from the original stories who hunted down and killed the Jedi. He is not the one who killed all of those kids. Vader is mostly mechanical. He of course has a light saber and knows how to use the force. We all know that size doesn't matter when using the force. So, much of the Big G's size is neutralized.
Godzilla is 40 stories tall and breaths atomic fire. Not to mention that Godzilla is incredibly strong. He can push down buildings and pick up subway trains with ease. He is mostly (nigh) invulnerable. He has only been defeated by an oxygen destroyer and runaway internal radioactivity.
What do you think people?