Oh oh, I like this match
I'll preface that my answer is Criston.
First, the Mountain. An almost 8 foot titan, likely the strongest human warrior that we know of, Jaime singles out his strenght as inhumane. He's also more agile than one would expect for someone of that size. And he's not some unskilled knight, far from hit. Bronn, whose ability is praised throughout the series and that would often be willing to fight under Tyrion's request, backs out from facing the Mountain without second thoughts, saying that one misstep would mean death. Gregor has a pretty terrifying reputation within Westeros. He gets defeated by Oberyn but Oberyn himself is one of the finest warriors of the age.
As I wrote earlier, Criston gets memed by some show only fans but from the books he was one of the absolutely greatest knights of his age. He symbolized "both the best and the worst qualities as a Kingsguard". Harwin Strong was considered the strongest man of his time and was nicknamed "Breakbones". At some point, Criston beat the shit out of him so bad that he got renamed "Broken bones" lol. He beat the Rogue Prince at melees. He really wasn't defeated in a 1v1 duel and died to arrows.
Not many people can face Gregor and best him. I think Cole has the ability to do so.