Goku would Crush Superman in a duel....

This is also why Batman is my No. 2 favorite, in spite of being DC. However, I don't think he could beat Superman, Hulk or Thor realistically.

He's got kryptonite on hand in case Superman goes rogue, and has used it against him before. And Superman once referred to him as "The most dangerous man in the world". In complimentary fashion.

Obviously he couldn't outfight any of them in a straight up brawl, but his greatest resource is his mind. He could easily figure out that Hulk is powered by gamma radiation and devise a way to dampen it or siphon it off (which has been used against Hulk before).

Thor...that's tough, as Batman usually refers matters of magic and gods to people like Zatanna and Wonder Woman. He's admitted that it's the area he's least comfortable with.