I'm new here... so this is my first post... but.

I think that it was Ben or Locke in the coffin.

Reasons for Ben:
Neither friend nor family. Death does odd things to people. Jack might have felt obligated to go simply because he was so interconnected with Ben. He fought hard at the beginning to not have him tortured when he was an imposter, and he performed surgery on him. He and Ben have an interesting connecting. The news of a death, since none of us are certain what the newspaper clipping says, could have been traumatic enough for him, in his depressed "I want to go back" state, for him to want to jump. It would also make sense that Kate didn't care to see him... although, if it was Ben, I would have expected slightly more vehemence from her regarding the death.

Locke
Neither friend nor family.... Locke seems to have come out of the good graces of everyone after joining the Others. It would make sense that Kate wouldn't care because she hasn't seemed tremendously attached to Locke one way or the other. The death of Locke would be enough, I think, to throw Jack over the edge... because they used to be close.

Oh well... I really have no concrete evidence for any of this. But one thing's for sure... I don't think it was Michael. His character has lost its' importance. I don't think his death would have had such a profound impact on Jack. Although, maybe I'm wrong.