Who should we tackle next?

Bariston did kill Maelys the Monstrous. And the pretender to the throne was supposed to be massive and exceptionally strong. From the pictures he might be a little bigger than the Mountain. Which means if Bariston did it as a young man with little experience, then I am sure as an experienced warrior who has barely lost a step he would be able to do it.

One thing I always found funny, was Ned assigning poor lord Barric the impossible task of killing the mountain like it ain’t no thing. For all the acts of bravery and sacrifice in the series, Barric deserves credit for taking on that suicide mission without complaint.

Janos Slynt's friends in the capital can defeat the Mountain

drogon could probably beat him

There is only one armour that survives contact with The Mountain: Plot armour.

Oberyn, obviously. In any fight in which he was aiming simply to win.

I think Robert Baratheon in his prime could

I did like how Bronn was humble enough in the show to question if he would be able to fight the mountain.

I think Greatjon umber is honestly such a beautiful and good inclusion on this list. He’s basically an upgraded mountain. Roughly same size, maybe even stronger, with tons more experience. The sight of him dominating the mountain makes me happy

Loras did defeat The Mountain at the tourney, however, showing guile by using a mare who was in heat to...ah...distract The Mountain's charger. Point is, Loras showed a mind to exploit one of The Mountain's weaknesses, not just going head-to-head with The Mountain's advantages.

Bariston, in the books, could have taken on anyone, except maybe Arthur Dayne.

I completely agree with the assessment of Ramsay. His greatest asset is being able to catch his enemies off guard with Shock and Awe tactics. In the show, he fought the Ironborn without a shirt and covered in scratches with a grin on his face, an intimidating sight for raiders who know that they are out of their element infiltrating a castle. However, these tactics would amount to jack squat against the Mountain.

Technically Oberyn did defeat him but got killed because he dragged it out.

What makes the Mountain really hard to beat is the amount of physical damage he can take and then recover in an instant to deliver a killing blow next moment just shocks people

I'd argue Syrio would be able to get the mountain.

I think The Blackfish in his prime could beat the Mountain. Even Jamie had a lot of respect for him as one of the best fighters in the seven kingdoms.

Bron's surprise weapon: anti-dragon crossbow

I am surprised the Hound isn’t on this list. Win or lose I would love to see Clegane Bowl II.

A rapier is actually a bit better than some longswords against an armored opponent, it's much easier to find the gaps in the helmet with a smaller point. Also, remember that no swords are meant for fighting against any armor. Even linen gambesons can stop the best swords.