Off the scale
One of these two was never ranked, so this cannot move a ranking it is not in.
The only thing on this page that ever solved Godzilla is not on this page's scale, and that is the entire point of both placements. It is a device. It does not fight, it cannot be provoked, and it worked once — completely, on the original, leaving nothing. What it cannot do is be compared: there is no second use to reason from, because the man who built it burned the design and went down with it. So the ranking has a hole in exactly the shape of the most effective thing in the franchise, and printing that hole is more honest than inventing a number for a chemical that has one recorded outcome and no opponent.
The Oxygen Destroyer killed Godzilla in Godzilla (1954).shown
the body disintegrated entirely; its inventor destroyed the design and himself with it
Kills Godzilla outright, reducing the body to a skeleton.
Serizawa destroys his notes and dies to prevent the weapon being recreated.
It has the only clean kill in this franchise. Dropped into Tokyo Bay it reduces a fifty-metre animal to bone in seconds, and its inventor destroys the research and himself alongside it so that nothing can rebuild it. No kaiju on this page has a result that decisive against anything. It is unranked because it is a chemical weapon rather than a combatant, and putting it in the list would be a category error the 1954 film is specifically about.
Conventional military force has no effect on him.
Listed at 50 metres, the original official height.
Fifty metres, the shortest entry on the page, and the reason every other one exists. He walks through Tokyo, the military does nothing to him, and he is then killed by a chemist who considers the cure worse than the disease. Every later incarnation is taller and none of them is more consequential — which is the plainest available demonstration that the official height measures the set, not the animal.
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.
Full receipts on the Godzilla ranking: The Oxygen Destroyer (2 citations) and Godzilla (2 citations).