Concerning Og, see Numbers Raba Hukat 32: ... that no strong one in the world arose that that was more fierce than he, (קשה הימנו)... And he remained from the mighty ones who were killed by Amrafel and his friends, (Gen 14)... And he was a husk of them, like peels of olives in the olive waste.
So from this saying, it would seem that Og was the strongest, and even he was put to shame by the mighty Refaim he hailed from.
Another instance can bee seen in Tanhuma Mattos 5: Two strong ones arose in the world. Samson from Israel and Goliath from the nations of the world. And both of them were lost from the world, etc... Why? Because their gift was not from Gd, rather they seized it for themselves.
Notice that the 'And' separates the two statements. In other words, the midrash is not saying that "there were two strong people who were lost from the world", It is saying "(1) There were two strong people in the world, AND (2) both of them were lost from the world."
It would seem that either Samsom or Goliath was the strongest person from this saying.
To offer a resolution of these conflicting Midrashim, perhaps all of the above mentioned were "the strongest" at one point in time. The Refaim were the strongest until they were killed in Abraham's time. Then Og took the title until Moses killed him. Then Samson was strongest, until he sacrificed himself to destroy the Philistines. Finally, Goliath was strongest until he was killed by David.
In any event, one can see that referring to someone as "the strongest" in rabbinic sayings is not meant in an absolute sense. Even if you reject my understanding of Samson and Goliath, you must accept the very plain inferiority given to Og as opposed to the Refaim, alongside Og's being called "the strongest".
I think that as far as finding an absolute winner goes, at best we have specific anecdotes to examine.
Using the above premise, Samson seems to be the winner thus far. See Sotah 9a, where Rav Asi says that He uprooted the two great mountains, Tzorah and Eshta'ol and ground them together. Even Og only lifted one mountain, as seen on Brachot 54b.
Genesis Rabah Vayehi 99:11, is also worth mentioning, though it may be less conclusive. It quotes Dan's blessing in Genesis 49:16, ...like one of the tribes of Israel, like the One of the world. Just as the One of the world needs no aid in war, so did Samson who arose from Dan need no others to help him, without a sword, rather with a donkey's jawbone.
So Samson essentially founded the term 'one man army', requiring neither outside assistance, nor sufficient arms to defeat overwhelming odds.