>>57190577Easily Mewtwo.
No pokemon comes remotely close while the others can at least get ditto’d.
There is no check or counter for Gen one Mewtwo. You paralyze the fucker and hope it doesn’t have rest and then go to war with your team against it or try to bring out your own Mewtwo.
You can’t even rely on any super effective attack since psychics are effectively a god type in Gen 1. Rayquaza’s needs an ability to rise to that godly status while Mewtwo’s power sat at the very foundational structure of these games. Admittedly I dropped after Moon and can’t speak on the power of legends from Gen 8 onwards but I doubt they showcase the sheer gap in power between itself and the rest of the entire existing Dex that Mewtwo did in Gen 1. The games literally were not ready for him and had to introduce two entirely new types to address the Psychic problem where Mewtwo sat as king.
No legendary has ever felt more powerful than Mewtwo in Gen 1 and they never will. It’s ridiculous to thing anything comes close.