The connecting record
shownA Storm of Swords
Gregor Clegane killed Oberyn Martell in A Storm of Swords.
Caught him by the leg from flat on his back, pulled him down, and smashed an armoured fist into his unprotected face, destroying the teeth and knocking the eyes out before he crushed the skull, shouting what he had done to Elia Martell while he did it. The thumbs into the eye sockets are the television staging.
What each of them is ranked on
The killer · Gregor Clegane · rank 2
the Mountain That Rides
Seven feet of him and more, nearer eight, carrying a six-foot greatsword in one hand and a shield on the other arm. Jaime Lannister, counting the men who could overpower him physically, arrives at both Clegane brothers and thinks the elder one's strength is not a human quantity. What settles the placement is that all of it happens inside somebody's point-of-view chapter: Ser Hugh of the Vale taken through an unfastened gorget in a joust, a stallion's neck half severed with one swing in a temper, a horse speared out from under him on the Green Fork with no effect whatever on the rider. Then a trial by combat he wins from flat on his back, full of holes from a poisoned spear, shouting what he did to Elia Martell loudly enough for the whole gathering to hear. Poison thickened with sorcery finished him in the end and took its time about it. The thing his brother later carried over a Red Keep wall was Qyburn's work rather than the man.
shownA Game of Thrones · ch. 30 (Eddard VII)
Stands well over seven feet, closer to eight, with arms as thick as the trunks of small trees.
statedA Storm of Swords · ch. 21 (Jaime III)
Jaime, thinking of the few men stronger than himself, settles on both Cleganes, and reckons the Mountain's strength like nothing human.
shownA Game of Thrones · ch. 62 (Tyrion VIII)
Wields a two-handed greatsword in a single hand while carrying a shield, and commands the van and left of Tywin's line on the Green Fork.
All 13 citations
shownA Game of Thrones · ch. 62 (Tyrion VIII)
Karstark spearmen kill his stallion under him in the first charge and he fights on unhurt.
shownA Game of Thrones · ch. 29 (Sansa II)
His lance rides up in his second joust of the Hand's tourney and goes through Ser Hugh of the Vale's throat.
shownA Game of Thrones · ch. 30 (Eddard VII)
Unhorsed by Loras Tyrell, he kills his own stallion with one blow of his sword and goes for the unprepared Loras until Sandor Clegane blocks him.
shownA Storm of Swords · ch. 70 (Tyrion X)
Fights Oberyn Martell as Cersei's champion in Tyrion's trial by combat, in heavy plate behind an oaken shield.
shownA Storm of Swords · ch. 70 (Tyrion X)
Wounded repeatedly at the joints of his armour and brought down, he takes hold of Oberyn on the ground, smashes in his face, and shouts that he killed Elia Martell and her son.
statedA Storm of Swords · ch. 72 (Jaime IX)
Pycelle judges the spear to have been poisoned; the wounds mortify and Tywin's order to save him fails.
statedA Feast for Crows · ch. 7 (Cersei II)
Qyburn concludes the venom was manticore poison thickened with sorcery so that the dying would be drawn out rather than quick.
shownA Dance with Dragons · ch. 65 (Cersei II)
Qyburn presents a knight eight feet tall in white enamelled plate as Cersei's champion, and the man carries her into the Red Keep.
shownGame of Thrones · S04E08 “The Mountain and the Viper”
Puts his thumbs into Oberyn Martell's eyes and crushes the head between his hands.
shownGame of Thrones · S08E05 “The Bells”
Fights Sandor Clegane on the stairs of the burning Red Keep, keeps coming with a sword through the chest and a blade through the eye, and goes over the broken wall with him.
The victim · Oberyn Martell · rank 9
the Red Viper
Won. That is the part everybody forgets about him, and it was not close: eight feet of ash spear against full plate behind an oaken shield, the joints found over and over, the hamstring cut once the bigger man had tired of chasing, the shaft snapped off in the belly. Gregor Clegane was on the ground and done. The afternoon only turns because Oberyn stood over him demanding he say Elia Martell's name aloud, and was put on his back for it. All of it is set down inside one point of view and filmed later almost shot for shot, which makes it about as well attested as any duel on this ladder — no chronicler, no gap of thirty years, nobody's grandfather remembering. No states are listed above this entry because there are none to list. He has no before and no after. His best quarter of an hour and his last quarter of an hour are the same one.
statedA Storm of Swords · ch. 38 (Tyrion V)
At sixteen he fought Lord Edgar Yronwood over the man's paramour; the duel was to first blood, both took cuts, and Yronwood's festered and killed him.
statedA Storm of Swords · ch. 38 (Tyrion V)
Rumour that he fought that duel with a poisoned blade is where the name Red Viper comes from, among friends and enemies alike.
statedA Feast for Crows · ch. 2 (The Captain of Guards)
Doran recalls him toppling much larger boys at the shoulder-riding game in the Water Gardens, and Oberyn promising on leaving Sunspear to do it again.
All 12 citations
shownA Storm of Swords · ch. 70 (Tyrion X)
Chooses light armour and a turned ash spear eight feet long for the trial by battle, and warns Tyrion the night before not to touch the weapon.
shownA Storm of Swords · ch. 70 (Tyrion X)
Wounds Gregor Clegane repeatedly at the chinks of his plate, severs his hamstring after the bigger man tires of the chase, and breaks the spear off in his belly.
shownA Storm of Swords · ch. 70 (Tyrion X)
With Gregor down, he refuses to finish him and demands a confession instead; Gregor trips him, smashes out his teeth and eyes and caves in his skull.
statedA Feast for Crows · ch. 17 (Cersei IV)
Cersei reports to her own small council that Gregor Clegane has died of the poison, months after the trial, and the cleaned skull is boxed for Sunspear.
statedA Dance with Dragons · ch. 38 (The Watcher)
Tyene Sand tells her sisters she knows the poison her father used, without ever naming it, and that if his spear so much as broke the Mountain's skin then Clegane is dead however big he was.
statedA Feast for Crows · ch. 7 (Cersei II)
Qyburn identifies it as manticore venom thickened by a spell to draw out the dying, over Pycelle's objection that manticore venom kills on the instant.
statedA Storm of Swords · ch. 38 (Tyrion V)
Struck Willas Tyrell's breastplate clean in a tourney; the boy's foot caught in a stirrup and his horse came down on him, crippling him for life.
statedA Dance with Dragons · ch. 38 (The Watcher)
Doran tells the Sand Snakes that Oberyn was ever the viper — deadly, dangerous, unpredictable — and that no man dared tread on him, while he himself was only the grass that hid him.
shownGame of Thrones · S04E08 “The Mountain and the Viper”
Circles the Mountain with a spear for the length of the duel, opens him repeatedly, brings him down, and is caught by the leg while standing over him demanding the confession.
What this does not settle
One death in one work. It is the only place the record puts these two in the same room, and it is evidence about that meeting and nothing wider — not a general matchup, not a rematch, and not a ranking of its own.
The ladder above it is argued from everything either of them has been shown doing; this page is argued from one citation.
Where the two disagree, the disagreement is the point, and it stays visible rather than being resolved by whichever of them happens to be on this page.