The connecting record
shownAvengers: Endgame · 2019
Thor killed Thanos in Avengers: Endgame.
on screen, and far too late
What each of them is ranked on
The killer · Thor · rank 15
Odinson — comics and screen
The baseline version is a god who has been fighting for a thousand years and it is nowhere near the top of his range. Rune King Thor gave up his second eye and hanged himself on the world tree to learn what the runes were, came down with enough of it to end the loop Asgard had been dying inside since the beginning, and then put the power away. The screen version's ceiling is a good afternoon at a forge.
shownThor · vol. 2 #80–85
Gives up his remaining eye and hangs himself on Yggdrasil for the runes, then unmakes the cycle of Ragnarök. — 2004
shownAvengers: Infinity War · 2018
Withstands the full output of a neutron star to forge Stormbreaker.
The victim · Thanos · rank 3
The Mad Titan — comics and screen
Everyone cites the Gauntlet. The Gauntlet is his second-best day. Holding the Heart of the Universe he erases all of existence, is talked out of it, and then puts the universe back exactly as it was including the people he had already killed. Then he erases himself, and Adam Warlock is the only one left who remembers any of it happened. The screen version does the smaller, better-told take on the same arc.
shownMarvel Universe: The End · 2003
Wields the Heart of the Universe, erases all existence, then restores it whole.
shownThe Infinity Gauntlet · 1991
Erases half of all living things with a gesture, then defeats Eternity.
shownAvengers: Infinity War · 2018
Completes the Gauntlet and erases half of all life across the universe.
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.