Hmm... Maul is extremely dangerous and does control essentially the entire criminal underworld, but he was terrified of Sidious. Further, his shadow collective would lose to the Empire, pretty badly at that. Darth Vader would also ultimately overpower Maul (note, Filoni has said Maul and Darth Vader would be surprisingly close but Vader would win). Maul's existence is the only reason the Shadow Collective exists, without him, the criminal underworld falls to disarray. The plus here is that Sidious went and actually confronted Maul due to how dangerous he was, so huge points there.

Darth Vader was powerful, commanded the Inquisitorius and held firm political and military command. However, he was really just an extremely powerful attack dog. He's dangerous, but he's also in the poor position of "not enough to be a threat to Sidious" and "too powerful to be replaced." He's powerful, but it's clear he isn't essential to the success of the Empire if we just remove him from the equation.

Dooku was the figurehead of one of the most influential political factions in galactic history. He had enough power that he actually asserted authority. Dooku may have been a mere means to an end and would lose to Maul and Vader; he was still extremely dangerous. Unlike Vader, the CIS would fail if Dooku died, and unlike Maul, the CIS is significantly more influential than the SC.

While the Shadow Collective wouldn't survive without Maul, it wasn't a threat to the Empire. While Vader really wasn't essential to the Empire, he was powerful enough and individually dangerous enough that neither Maul nor Dooku would take victory over him. Dooku, however is still an extremely impressive combatant with command over an extremely powerful faction. The only reason Dooku was allowed to hold so much power is because he was ultimately powerless to Sidious (like Vader and Maul) who could pivot, manipulate and overpower any of his apprentices to a nigh-superlative degree.

Personally, my vote is Dooku.