10. Robert Baratheon
"He was a good man, a great warrior. And a terrible king."
It can be difficult eradicating the image of the overweight, drunken, whore-loving King Robert of Game of Thrones first season from our minds.
Before the events of the show, the stag was once one of the most feared warriors in all of Westeros, inspiring terror in the hearts of his enemies with the sight of his crushing warhammer. It must be remembered that to create his comfortable existence where he could drink jug-after-jug of King's Landing wine and scoff banquet-upon-banquet, he first had to seize power through conquest (though this conquest would prove far more satisfying to Robert then actually ruling).
It was during Robert's Rebellion that he was able to cement his legend as one of the greatest warriors ever, vanquishing another candidate for that coveted title - Rhaegar Targaryen - at the Battle of the Trident.
A clash between Robert Baratheon and his former subject Gregor Clegane - before the days when Robert had ever even heard of a breastplate stretcher - would be a fascinating one. Ostensibly, The Mountain has an advantage - both warriors' strength is their insane power, but you would expect the gigantic elder to Clegane be stronger even than Robert.
However, Robert - in his younger years at least - would certainly have had a speed advantage over Gregor Clegane and his having fought the likes of Rhaegar Targaryen in one-on-one combat would give him an edge in experience. Moreover, Robert has been used to fighting (and defeating) quicker, more agile opponents so he would easily be able to connect his warhammer with Clegane's oversized frame.