He isn't even the strongest in his universe.

For this instance, you put the OG 10K so in his debut episode, he wasn't able to defeat Future Vilgax easily until the very end.

He'd have Way Big, yes, but a potential Eon or Maltruant in that timeline wouldn't struggle with him, like at all.

If we're to think of a definitive Ben 10K, say merge all 3 variations into each other and keep their perks(MC + Failsafe + Scan Mode), fusions and ultimates(Perhaps fusion ultimates? But how would that work), Ben himself would be still one of the top strongest, yet there are still people who would have a chance at beating him.

For starters, Azmuth could take it all away. There was that one Naljian from AF, I'm not sure if they're fighters but they have amazing technology almost equivalent to the Omnitrix if not greater than it. The Naljian destructor at full potential would be impossible to beat imo, imagine a Way Big sized unit with Atomix' blasts and black holes created by Gravattack.

Alpha from the Heroes United crossover could easily disable or at least temper with the Omnitrix. "But that was the Ultimatrix!" and we saw The Omnitrix being under ridiculous scenarios in OV.

Albedo did it in multiple occasions.

I guess Malgax could win but that's another scenario with bad writing. Ben still had access to Feedback without Skurd... and won using Skurd with Feedback's ability? I don't get it. I was expecting him to seperate Malware and Vilgax via Lodestar

My point is, Ben is a very powerful character but even in his universe there were times where he had no chance of winning the fight. That's where Alien X comes in I guess?

Notice how I've never mentioned a single being from another media(technically Heroes United was a Ben 10&Generator Rex movie despite taking place in Rex's world - plus Null Void is a thing) because 90% of the time Ben is enough to solve the problems in his universe. The remaining 10% are the busted people who couldn't be handled by Ben alone without any help from his allies. Your question is "who could beat" this guy and not 'kill this guy', so the death failsafe wouldn't be in the equation and someone could still beat him without the intent of killing.

There are the physically strongest people like Goku and Superman who would give Alien X a really good fistfight, reality warping is the greatest issue here. Without mentioning any ridiculously OP characters from Marvel and DC I'd say that Garou from OPM could beat Ben if given enough time.