The Mountain(ASOIAF) vs The Mountains(GOT)
Straightforward enough. The Mountain from the books is displeased with his representation and challenges the 3 different Mountains to a 1v3 battle to the death. Each Mountain starts out fully armored and with swords proportional to his height. Book Gregor gets his shield since the Show Gregor seem to go shield-less.
Fight takes place in an abandoned Six Flags amusement park.
Book Gregor is bigger than all three of his show incarnations and more heavily armed and armoured. Ian's Mountain would be a simple matter to kill, not only due to his less imposing build, but due to how relatively light his armour is.
Conan's Gregor is more skilled than Bjornsson's, who mostly flailed around like a butthole, but book Gregor benefits greatly from his greater reach and the fact that he uses a shield. Bjornsson would likely be the second easiest to kill since his armour is not as heavy as Conan's.
None of the show Mountains are described as being any faster than they look, whereas book Gregor is described as being deceptively quick for his size. He also has better battlefield feats, charging through pike lines and crushing them on foot after his horse died.
Book Mountain wins.