Ultraman gave Earth 1 and Earth 2 Superman some problems, but his universes Luthor handled him easily, and it's not like Ultraman beat them silly or clearly dominated.
Mongul, on the other hand, tanked an extended beat down from Supes and beat him so badly he had trouble even standing up some time after the beatdown.
For starters, he made him intangible so he couldn't harm anything. From there it was mostly off panel, but it was pretty obvious Ultraman was outclassed by the tech his worlds Luthor was using.
And yeah, Ultraman could be tough, but Mongul was basically treating PC Superman like a child. IMO, the one time Superman beat him is kind of PIS, even though Supes fell into unconsciousness himself from the effort..
Ah, it was tech, then. Luthor (PC) would be around Reed Richards level, IMO. So, it's a no brainier. No pun! Anyway, Ultraman broke Superman's jaw once.
Lex Luthor and Alexi Luthor were in the same story, and Ultramans Luthor came off well by comparison. The two main Luthors actually traded Supermen, using their own ingenious devices, but both failed, while Ultramans Luthor succeeded..
Although to be fair, Earth 1 Superman is a genius himself, while Earth 2 Superman had help from Lois. Lex Luthor had him dead to rights with a shrink ray that keeps shrinking, until Lois beat him up and stole the device, saving him.
I always thought Alex Luthor was better than regular Lex Luthor. Alex brought back the Multiverse for crying out loud. Which is better than anything any genius in any universe can do. Even Reed, IMO.
That's Alexander Luthor you're thinking of. He's not Alexi, who was native to Earth 2 and killed by Brainiac in COIE when he complained that Lex Luthor was being given too much respect.
The guy was completely insane, trying to wipe out both Earth 1 and Earth 2 when Lex and himself failed to defeat their Supermen.
Even Lex Luthor was appalled by how bloodthirsty he was.
As we discovered in Infinite Crisis, Alexander is pretty insane himself, but in his defense he went from being a little boy during the Crisis to growing up near instantly, to living most of his life in a glorified box...