... huh. Well Pre-Crisis has a lot of hax, Post Flashpoint resists all of them, and nothing Post-Flashpoint has really affects Pre-Crisis. They’re about the same experience wise, Post-Flashpoint having better combat skills, but Pre-Crisis has arguably far higher AP, though Post-Flashpoint AP can be quantified to be even higher.
Hilariously they both have feats that kinda counter each other, Pre-Crisis punched a guy so hard he recreated a Multiverse, Post-Flashpoint punched a guy so hard he destroyed a Multiverse, Pre-Crisis scales to injuring the Anti-Monitor, Post-Flashpoint scales to injuring World Forger (both being brothers)
Guess it comes down to how long the Sun Dip can last and if Pre-Crisis can send him through time. If the Sun Dip is temporary then Pre-Crisis wins by pure stamina and being consistently that level. So I’d lean towards Pre-Crisis like 6/10, really close but the BFR by time travel could mess Flashpoint up.
Pre-Crisis Superman wins, sundipped Rebirth Supes hurt Forger but his amp probably faded away after (otherwise Superman could solo the Legion of Doom) so it all depends on if Rebirth Supes can land a hit. Otherwise Pre-Crisis Superman slams him with all his might knowing he can take it since he wears the S, and knocks him out of the universe, not to mention all the hax he has over Rebirth Supes like casual time travel and super hypnosis.