Superman - 10
Thanos - 15 head shots before falling.
Juggernaut - he would take about 10 before falling
Thor - at his best, 10 to 12 head shots. At his normal self, can't tell ya.
Wolverine - 3 Hits and is koed.
Mr Fantastic - One hit and dies.
Hulk - how Hulk has been portrayed lately, he would be the last to fall. Hell, he could probably just stand there and giggle while they pounded on him.