For me this answer is easy,Sugar Shane Mosley is easily the biggest threat to Pacquiao,first of all Shane knocked out way bigger men then Pacquaio,he knocked out Ricardo Mayorga who basically fought Shane as a light heavyweight at the weight of 170,shane has very good power!!! he also has great speed and a indestructable chin as well. I just see Mosley being too much for Pacquiao to handle.....Anyway Who do you feel is the biggest Threat?
Who ever catch him cleanly to hurt him, as simply as that every fighter has a change only one fighter has more chance than others but still they have a chance.