E.G. Did Darth Sidious ever think: "I am the greatest Sith ever, even better than that Valkorion dude years ago."

Sidious did label himself as the most powerful Sith ever. Which is canonically true.

Plagueis also knew Vitiate, and thought that he was the most powerful- if there had been someone more powerful than him, that guy was unknown to him. The modern Sith Lords kinda compared themselves, yeah.

This probably won't end well.

@thekinfing:

Why?