Maul was very skilled with lightsabers, but he did not seem to have been adopted by Sidious for his intelligence and autonomous thinking (as far as one can judge by what we briefly saw in the movies, but admittedly, this remains speculative because he was not very... talkative!)
Ventress wasn't a full-fledged "apprentice" of Dooku, properly speaking, and, moreover, she was not composed enough and coldly calculating enough, unlike Dooku.
Anakin Skywalker/Vader was very powerful in mastering Force techniques and in wielding a lightsaber, but he was not a calm, composed and far-seeing person, and his powerful but overwhelming emotions could serve him well but they also frequently clouded his judgement (just think of the way he reacts on various occasions by just dashing into danger like an unrestrained tornado -- and think of his last move in his fateful duel against Obi Wan Kenobi : what he did was just plainly stupid!! ::) ::) ::) He could have jumped everywhere else around Obi Wan, or even wisely adjusted the amplitude of his jump, or circumvent the obvious danger by gradually gaining the "high ground" or tricking Obi Wan by letting him think that he was "losing ground" while actually letting Obi Wan exhaust himself and waiting for the right moment to strike, just as Dooku did in his first duel with Obi Wan....Think of it : Dooku was the only one to be able to repeatedly defeat Obi Wan : Why? - because, unlike others, he perfectly knows that pretending to lose ground or to hesitate, and using tactical elements of surprise is as much important in a duel as resorting to raw acrobatic skills. I admire that and I can testify to the fact that it is crucial in duels :) ;) )
... thus in the end, I voted for Dooku, because he is both an excellent dueller and an expert in telekinesis, and moreover, he is an intelligent, composed and tactical mind and, paradoxically, an "idealistic" person who turned to the Dark Side because of his political and ethical convictions (I like this idea). And to all that, you must add that he had the logistic and material means to fulfill his ambitions (he was of noble descent and had a tremendous wealth).
First point : Dooku is the best dueller and has a lot of stamina (for a person of his age). Just think of the way he managed, in Episode 2, to withstand a continuous and demanding three-round fight (first Obi Wan, then Anakin, and finally Yoda!) and of the way he managed to defeat two of the most powerful Jedis at that time (Anakin - he severed his right arm and knocked him out- and also Obi Wan - he incapacitated him by inflicting him two bad gashes, one on the shoulder and the other one on the thigh, and he most certainly would have finished him if Anakin had not parried his strike). And on his second encounter with the Anakin-Obi Wan duo, he again managed to knock out one of the two Jedis (Obi Wan) while holding his ground against the other (Anakin) until his opponent resorted to a skillful, unconventional and twisted move (in the end Dooku was defeated by a trick and what was fatal to him was both his overconfidence and his failure to see that his nostalgic respect for some of the old-fashioned conventions of aristocratic duels (not trying to hold/restrain your opponent's body with your bare hands while blocking his blade, which was seen as a rude and disgusting move unworthy of an aristocratic fighter - only quick blows were allowed) were not shared by Anakin.
And, by the way, when you compare the duelling skills of Dooku to those of "old" Obi Wan and Vader (when they meet again onboard of Death Star 1 ::) ), you can clearly see that aging is not a valid excuse for lack of training! (on Geonosis, Dooku was already MUCH older than "old" Obi Wan and "old" Vader when they """""""fought"""""""" onboard of Death Star >:( ::) )...
Second point : Dooku, although far less powerful than Sidious, was one of the best practitioner of telekinesis of the Jedi Order, and he seemed to be very skilled in using various techniques (electromagnetic Force lightning, distal psychokinesis, most certainly mind control and hypnosis).
Third point : Dooku is the "Bruce Wayne" of the Dark Side (lol -poor comparison, forgive me) : his powers lie as much in his physical prowesses as in his formidable wealth (which enabled him to bribe many people, to buy abundant war material, to raise a separatist army, to finance a few innovative experiments on cybernetics, among which was the reconstruction of a well-known Kaleesh general... etc)
It is interesting to note that each one of Sidious' apprentices was seduced by the Dark Side by a different kind of trick.
Darth Sidious/Palpatine adapts his speech and technique to the person he wants to seduce, and to what his/her dominant passion and prominent weakness are : Darth Maul was raised in the Dark Side of the Force by means of his fear of insecurity as a young child, his agressivity and his need for guidance. Count Dooku was convinced into it by means of his pride, his resentment and his political ambition. Anakin Skywalker was tricked into it by means of his passionate temper, his rebellious attitude, his paradoxical longing for a father-like authoritarian figure, and his overwhelming fear of losing the ones he loved. Finally, Luke Skywalker would have been trolled into the Dark side by means of his anger towards his father, his attachment to his friends, and his hatred against the Empire. There are many paths towards the Dark Side, and many possible temptations…
And all that was planned and done by a cunning and manipulative Sith Lord who managed to patiently and gradually rise to power by raising rival political forces against each other and by exploiting the fears and desires of his allies and enemies as well! Palpatine/Sidious truely is … insidious, and he is the perfect embodiment of what Dante Alighieri , in his Inferno, called the Fraudulent and Treacherous souls (the last and deepest circles of the Inferno)… :o :-\