Sasuke pain (having his parents murdered in his presence, losing Itachi and finally Learning the truth about Itachi)
Naruto pain ( growing up without parents and being a jinchuriki )
Obito pain ( losing Rin to Kakashi and losing fate in the real world)
Nagato pain (having his parents killed before him, Yahiko dying in his hand, and being used by Madara)
kabuto pain (being an homeless spy)
you can add to the list but I think Sasukes pain is the hardest.
Naruto pain ( growing up without parents and being a jinchuriki )
Obito pain ( losing Rin to Kakashi and losing fate in the real world)
Nagato pain (having his parents killed before him, Yahiko dying in his hand, and being used by Madara)
kabuto pain (being an homeless spy)
you can add to the list but I think Sasukes pain is the hardest.