Strongest incarnations of Superman:
Superman Prime One Million after the 15,000 year Sun bath
(only assumed to be the strongest though, as he's completely featless)
Thought Robot Superman
It took the combined energies of the Anti-Matter Ultraman and the positive matter Superman to signal and power it. It took the form of a giant Superman while controlled by Kal-El. The robot fought and defeated Mandrakk, discovering his original identity in the process. The battle left him blinded and damaged beyond repair. Before he falls, the robot engraves its own tombstone with a warning.
Pre Crisis Superman with Sword of Superman
Multi Galaxy buster
Batman with a million years of prep....
beats none of them.
I should also note, Superman is NOT the strongest character in DC. Not even close. People always seem to have that notion. Spectre, Mister Mxyzptlk, Lucifer, Michael, The Presence, COIE Anti Monitor all stomp him into complete dog shit.