The connecting record
shownDigimon Adventure: Our War Game · 2000
Omnimon killed Diaboromon in Digimon Adventure: Our War Game.
What each of them is ranked on
The killer · Omnimon · rank 6
Omegamon — two characters in one body — adventure
The franchise's flagship, and its most awkward ranking problem: Omnimon is not a character but two of them, fused. What it did is not in doubt — it is what stopped Diaboromon with a global clock running and a live missile in the air. It stops at this rank because every appearance since has established it as the answer that gets sent, not the question nobody could answer.
shownDigimon Adventure: Our War Game · 2000
Formed from WarGreymon and MetalGarurumon and destroys Diaboromon before the missile lands.
statedDigimon Reference Book · Omnimon entry
Listed among the Royal Knights.
The victim · Diaboromon · rank 11
Doubling — adventure
Its attack is arithmetic. Diaboromon copies itself across the network faster than anything can delete the copies, and by the time anyone understands the problem there are too many to fight — so it launches a nuclear missile at Japan and lets the clock do the rest. Beaten, but the thing that beat it had to be assembled specifically.
shownDigimon Adventure: Our War Game · 2000
Replicates across the internet beyond any capacity to delete it, and launches a missile at Japan.
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.