The connecting record
shownOne Punch Man (manga)
Saitama killed Lord Boros in One Punch Man (manga).
Manga only. In the webcomic Metal Knight recovers the body and Genos finds it in a tank, and the source does not say whether he survives.
What each of them is ranked on
The killer · Saitama · rank 1
Caped Baldy — Hero Association register
Aced every physical test at the certification exam and scored 71 out of a pass mark of 70, which put him at the bottom of the lowest class in the organisation. Punched apart the meteor falling on Z-City, killed the Deep Sea King in one blow in front of the heroes it had already gone through, and met a beam aimed at the Earth's surface with a Serious Punch whose shockwave parted cloud cover across the planet.
statedOne Punch Man · ch. 16, pp. 1-8
The certification exam awards him 71 points against a pass mark of 70 and a C-class designation.
shownOne Punch Man · ch. 16, pp. 1-8
Takes the physical tests at the exam centre in front of examiners and rival candidates.
statedOne Punch Man · ch. 17
Genos, reading the registry, says the two of them have been placed at the lowest rank of S-class and of C-class respectively.
All 11 citations
statedOne Punch Man · ch. 21, pp. 1-4
Genos tells him he has been moved from C-class rank 388 to rank 342.
shownOne Punch Man · ch. 21
Punches through the meteor falling on Z-City and walks away.
shownOne Punch Man · ch. 28
Kills the Deep Sea King with a single blow.
shownOne Punch Man · ch. 36
Repels Boros's Collapsing Star Roaring Cannon with a Serious Punch.
shownOne Punch Man · ch. 36, pp. 54-57 & 62-63
The shockwave off that punch parts the cloud cover across the whole planet.
statedOne Punch Man · ch. 36, pp. 68-69
Boros tells him he had strength to spare and was still holding back.
statedOne Punch Man · ch. 177, p. 37
Is promoted to A-class rank 39 after the Monster Association incident.
statedOne Punch Man · webcomic ch. 96, p. 16
The webcomic gives the same A-class rank 39 placement.
The victim · Lord Boros · rank 3
Crossed space on a prophecy that promised him somebody worth fighting, then absorbed three ordinary punches, a run of Consecutive Normal Punches that pulled him into pieces, and last the shockwave off a Serious Punch. He regenerated from all of it bar the last. Ranked on what he took rather than anything he landed. The webcomic does not kill him, which is the one place on this ladder where the two versions disagree about an outcome rather than about a cast list.
shownOne Punch Man · ch. 34, pp. 12-13
Survives Saitama's first punch, to Saitama's visible surprise, and his sealing armour breaks apart.
shownOne Punch Man · ch. 36, pp. 7-25
Knees Saitama off the ship to the Moon, and Saitama jumps back and lands on the ship again.
shownOne Punch Man · ch. 36
Is torn apart by Consecutive Normal Punches and re-forms almost at once.
All 7 citations
statedOne Punch Man · ch. 36, p. 29
Says the Collapsing Star Roaring Cannon releases the whole of his latent energy and can scour the Earth's surface.
shownOne Punch Man · ch. 36, pp. 54-57 & 62-63
Fires that beam and is critically damaged when the Serious Punch repels it, unable to regenerate any further.
shownOne Punch Man · ch. 36, pp. 67-69
Dies on the wreck of his own ship after congratulating Saitama on the fight.
shownOne Punch Man · webcomic ch. 150, p. 18
In the webcomic he is found unconscious but alive in a recovery chamber in one of Metal Knight's laboratories.
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.