What is the highest power game you could run, spanning from games about mice and insects to games where beginning characters can punch universes into dust.

Actually, i'm curious. Let's say DND has the most extreme power curve ever. Let's say you start as a normal human at Level 1, and at Level 30 or whatever, you become able to destroy infinite multiverses with a wave of your hand.

'What stories would you even be able to tell' stories about gods making and destroying multiverses out of spite like the gods of mythology only on crack. stories about growing from weak little man to guy who punches out elder gods every single day to make the universe a safer place for humanity. stories about a god-killing assassin. just take any plot and turn it up to a million. read fiction like dc, marvel, and gurren lagann to get inspiration.

I know you didn't specifically mention TSR era D&D. But I provide this as an example of how high level doesn't nessisarily mean killing gods.
In TSR era D&D, the maximum level was 36. This was for pcs as well as for npcs.
In AD&D 1st Edition, it's true that the gods had stats. And players took advantage of that thinking that if it had hit points, it can be killed. This was not the intention of the developers.
In AD&D 2nd Edition, RAW/BTB, killing gods was no longer on the table. They no longer had stats. The best you could do is kill one of their avatars.
The adventures for characters between 20th level and 36th level were centered around going to the other planes of existence, questing for legendary artifacts, advising rulers and kings, or becoming rulers and kings themselves. They were undoubtedly the movers and shakers of the world. But not God killers.

Just play Rifts with no restrictions on character creation.

Godlings, Cosmoknights, Mega-Heroes, homeless people (Vagabond OCC) and everything in between.

Oh, and bring in the SDF-1 because a 3600 foot long humanoid battle fortress with a thousand attacks per round and a main gun whose thousand foot deep impact craters are the size of counties is a Tuesday for Rifts nonsense.

The Mechanoids have motherships the size of continents and eat planets for fuel. Lovecraftian Intelligences that exist simultaneously across multiple realies rule Megaverse^tm spanning empires.

There is nothing too powerful or crazy to be out of place in Rifts.

Our player group in Rifts ended as the rulers of a couple planets in Phase World, protected by their military comprised of clone Zentaedi warriors withe SDF-4 Liberator flagships, Icazuchi cruisers, and enough VeriTech squadrons to fill them.

Bear in mind, that made them a minor nation in Phase World.  The big three nations can overpower them