The connecting record
statedundoneA Storm of Swords
Gregor Clegane killed Beric Dondarrion in A Storm of Swords.
A lance through the chest at the Mummer's Ford, which is the first of the seven deaths and the one every later scar is measured from. Thoros of Myr fails to save him, he dies in an ash grove that night, and the last kiss at his own funeral puts him back up. Gregor is later said to have killed him a second time, with a dirk through the eye.
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 · Beric Dondarrion · rank 23
the lightning lord
Rank a fighter at his best hour and Beric Dondarrion is the case that turns the rule inside out, because his best hour arrives after he starts dying. Alive, the novels never show him win anything at all — a name in the procession at the Hand's tourney, a lordling given a hundred men because his rank suited the commission rather than his skill at arms did, and then an ambush in the riverlands that kills most of the party and leaves nobody certain he came out of it. The famous version is built entirely out of the damage: one eye gone, the skull crushed at the temple, a noose scar, a wound clean through the chest, and a sword he sets burning by opening his own palm. Thoros of Myr has brought him back six times by then and says a seventh would finish them both. He still loses the only duel a reader is shown, opened shoulder to breastbone by Sandor Clegane, and he still gets up. On screen the last useful thing he does is hold a corridor full of wights so a girl can reach a door, and it costs him the death that sticks.
shownA Game of Thrones · ch. 29 (Sansa II)
Rides in the procession at the Hand's tourney, where Jeyne Poole announces she would marry him on sight.
shownA Game of Thrones · ch. 43 (Eddard XI)
Eddard Stark refuses Loras Tyrell the commission and gives it to Beric instead, putting a hundred men under him to bring Gregor Clegane to the king's justice.
statedA Game of Thrones · ch. 55 (Catelyn VIII)
Tywin Lannister's army ambushes that party and kills most of it, and word reaching Robb Stark is that Beric may have escaped but nobody knows.
All 12 citations
shownA Storm of Swords · ch. 34 (Arya VI)
Arya sees what is left of him — one eye gone, part of the skull crushed in, a noose scar at the throat and a terrible wound through the chest.
shownA Storm of Swords · ch. 34 (Arya VI)
Cuts open his own palm with the blade to set it alight, then loses the fight when Sandor Clegane splits him from shoulder to breastbone, and is on his feet again before the Hound has left.
statedA Storm of Swords · ch. 39 (Arya VII)
Asks Thoros how many times he has been brought back; Thoros answers six, and says the seventh may be the end of them both.
statedA Storm of Swords · ch. 39 (Arya VII)
Can no longer remember his own past well enough to find his ancestral seat, and Thoros takes this as proof the Lord of Light is not done with him.
statedA Storm of Swords · ch. 43 (Arya VIII)
The ghost of High Heart calls him the Lord of Corpses because the stink of the grave on him is fresh.
statedA Feast for Crows · ch. 42 (Brienne VIII)
Thoros tells Brienne that Lord Beric's fire has gone out of this world and a grimmer shadow leads the brotherhood in his place.
shownGame of Thrones · S03E05 “Kissed by Fire”
Lights the sword with his own blood, sets Sandor Clegane's shield burning, and is cut down through sword, armour and shoulder before Thoros prays him back.
shownGame of Thrones · S07E06 “Beyond the Wall”
Sets a resurrected snow bear alight with the flaming sword beyond the Wall, and afterwards cauterises Thoros's wounds with the same blade.
shownGame of Thrones · S08E03 “The Long Night”
Loses the flaming sword and is stabbed repeatedly covering the retreat of Arya Stark and Sandor Clegane through the castle corridors, and dies once the great hall doors are shut.
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.