Well the Hulk has shown to overcome energy blast, magic, mystical force fields and so forth. Based on encounters only the Juggernaut is posing a significant threat to him that he could stand toe-to-toe with the Hulk. But then again Juggy has many weaknesses (magic, telepathy attacks, mystical energies) and Hulk's strength is limitless. In my opinion, based on the Hulk's abilities if his strength increases to levels not explored before he could very well pound the Juggernaut until that cytorak crystal comes out of him turning him back to cain marko-just like onslaught did. So Thanos can be defeated by Hulk's strength as well.