It's like in Naruto, Pein/Nagato is/was arguably the most powerful living being but is that really important?

It's actually Kaguya but I get your point.

Windu and Dooku (I guess ending in 'u' means something here) are both the most ingenious saberwarmen

You're probably right about that, in terms of pure skill they were the best swordsmen I can think of.

Both arguably invented their own style but it's more that Dooku did while Windu mastered a style that was already invented who barely anyone else could master.

Here you are off. Windu used Vapaad and Dooku used Makashi. Makashi is an ancient style which was designed specifically for dueling, it emphasizes finesse and precision but it's weak against a heavy and direct assault which is why Anakin beat him with his dark side augmented form 5 attack. Windu used a variation of form 7 which feeds off of an opponents aggression whereas classical form 7 is mostly used by sith and relies on ones own aggression to produce a chaotic and eclectic offense.

If we talk raw power, though, it's gonna be probably an OG Jedi or Sith but frankly that's not really important to me.

The most popular answer among those who know SW lore is Luke Skywalker, in the EU he became OP as fuck but much of it is no longer canonical thanks to Disney. In my mind though, tact and skill are a form of power in and of themselves so the power of raw exertion and force is not the only thing that counts.

I think the fact that Anakin opportunistically 2v1'd both to get rid of them shows what a coward he was deep down. An efficient coward, but one nonetheless.

I wouldn't say Anakin was a coward, in fact he was extremely arrogant and thought he could take on anyone including Yoda and Sidious. Obi Wan was weak against Dooku because he was a style 3 specialist and Dooku's style 2 is specifically designed to snake and weasel it's way around a strong defense. Anakin has also faced Dooku alone multiple times during the clone wars.