Superman has somehow ended up in the Marvel Universe.
Marvel's "One Above All" don't like it one bit so He/She/It dispatches 1 being to kill superman. This is not to be a battle/brawl or any kind of struggle. This being will simply show up and kill Superman. Who get's it done?

Tier 1

Tiamut
Owen Reece (as depicted after the original secrete wars)
White Phoenix of the Crown
Protege

Tier 2

Adult Franklin Richards
Korvac
Arishem the celestial
Walker

Tier 3
The Vishanti ( Osthur-Hoggoth-Agamotto)
CH Thor
Demogorge

Tier 4

The Inbetweener
Dormammu
Stranger
Classic Grandmaster