There's a lot of Supermen to consider...and most of them have already been named from the Quantum Superman to Trinity Superman.
So....I'll just list some of my favorite one offs and amalgams. In no particular order of power...
1. Super Martian (Earth 32) Supes & Martian Manhunter
2. Super Demon (Earth 17 & Earth 13) Supes & Etrigan
3. Super Shazam (Action Comics #768) Supes with Capt. Marvel
4. Super Doom (Nu52) Supes possessed with the Doomsday Virus
5. The Kryptonian (Fantastic Four Crossover) Supes with Power Cosmic
6. Bronze Age Giant Superman (Earth-1 & Earth-2 Superman Fusion)
7. Green Lantern Superman (Earth 32)
8. Supermage (Earth 33)
9. Supreme Superman (Legion of Superheroes Superman X and Superman from the past fusion)
and lastly
10. Thor-El (Amalgam Crossover, Electro Blue Superman fused with Thor)
Aside from these official amalgams, in terms of powerful Superman incarnations the Infinite Sized Superman from Superman Adventures #47, where he grew so large he dwarfed the observable Universe.
Also in terms of power ups, any Superman wielding the Bronze Age Sword of Superman or stating the term "Zyxma" in the Golden and Silver Ages gained the power of a 5th dimensional Imp on top of their existing powers. So there's always that.