It must Ben in the coffin.
1. Who else would have nobody show up at their funeral? Ben had spent almost his entire life on the island, we know his family is dead and most likely he didn't know anyone in the real world.
2. When asked by the funeral director if he was friend or family Jack relied, "Neither." Who else would he say that about other than Ben?
3. When Kate reacts to Jack telling her about it she responds, "Why would I go?" Obviously this is someone Kate doesn't care for - Ben?