Count Dooku and Mace Windu Vs Darth Sion and Meetra Surik(The Exile)
Darth Sion a unstoppable force of power and ferocity, who learned how to survive the most painful of attacks by calling on his anger pain and hatred, a powerful and aggressive Sith lord Sion showed his power when he took on the Jedi Exile on Korriban and later on Malachor where the exile would of lost had she not used Dun Moch and eroded his will
Abilities. there really isn't much on sion - force wise anyways - besides his immortality technique and some knowledge of dark healing which isn't much.
Form.Given the fact that he was able to fight Meetra Surik on even grounds(despite her being able to learn her opponents style by simply observing them) and his surviving not only the downfall of Exar Kun's empire but also Revans empire(with latter's empire going on to kill eachother, the fact that he survived Korriban while impressive is to be expected) given his aggressive personality and many years of experienced( a la Darth bane) I believe we can safely assume Sion had Some if not a lot of knowledge in the Juyo form and knew Makashi to some extent(which was quite common during his era)
Meetra Surik. a powerful jedi whose effort helped bring back the jedi order and the republic from the brink of extinction, as a young jedi she followed her best friend Revan's(whom she considered her brother)path and fought along side him on various occasions, her love and loyalty to Revan where so strong that he when ordered that the Mass Shadow generator be activated she did so without question, after the destruction of malachor the echo was so strong that she had to cut her self of from the force in order to survive, she would later go on to defeat the Sith Triumvirate and save various planets from the sith. after the defeat of Darth Traya, she later traveled to the unknown regions in order to rescue revan and defeat the sith emperor Vitiate who held him captive.
Abilities. A powerful jedi master who during her return from exile would go on to learn various techniques from both the gray jedi Kreia and her companions, such techniques include- Battle precognition, Two different forms of meditations from both Mical(the disciple) and her droid T3-M4, and how to become slightly resistant of force powers from Atton rand. as a force user Meetra was highly talented and many force powers including but not limited to, Force whirlwind, Force Valor, Force Resistance, Battle meditation and even rare techniques like Breath Control and Beast Trick.
Form. Meetra was highly proficient in Shii-Cho, Makashi and Soresu and was able to show her lightsaber prowess when she defeated Darth Sion, Darth Nihilius, Atris(a Juyo user) and Darth traya who I would assume preferred Niman. despite all that however, Meetra had a special gift that allowed her to learn her opponents form by simply observing them and later applying in the form like an expert
Count Dooku. a powerful jedi master turned Sith Lord who became the leader of Separatist droid army after he joined the dark lord Sidious, as the dark lord Tyranus he not only demonstrated his light-saber prowess when he took on Obi-Wan, Anakin Skywalker, Yoda and various other jedi masters and apprentices but also his brilliant role as a tactician for the Separatist army.
Abilities. Dooku was highly proficient at telekinesis often times used it as means of flight, he also had knowledge of Force Choke and was highly skilled in Force Lighting having demonstrated his skill in it various times, he also knew the rare Beast Trick technique and was even able to bend the will of a krayt dragon
Form. While Dooku was a Master of Makashi, by the end of his career as a jedi there was little he didn't about the lightsaber froms(the only form he didn't know was dun moch) and even had enough know how of the forms to teach grievous all the form, despite that he heavily relied on Makashi which was only effective against jedi and while he was knowledgeable of all forms he never really mastered any other from besides Makashi.
Mace Windu. a powerful Jedi master second only to the grand-master Yoda, was a Jedi general that lead the republic to victory on many occasions(one time he alone took on an entire platoon of droids on dantooine?) and was able to fight Sidious on even ground(although he had to use vaapad to its fullest extent) before Anakin Skywalker turned to the dark side and killed him
Abilities Mace Windu was a powerful jedi who - thanks in no small part to his form Vaapad - was able to use the darkest powers of either the darkside and lightside his powers include, Force Wave, Force Grip, Force Crush, Force Speed and was highly proficient in telekinesis.
Form. Windu was a master of all seven forms and was counted among one of greatest duelist of his time, not only was he a great lightsaber duelist but he was also a great martial artist(as demonstrated on dantooine) but he also created his own Lightsaber form which was like juyo but with a few exceptions, to learn the form he created - Vaapad - the learner had willingly embrace the dark-side(but not fall) and the thrill of battle, his form was so near the dark side, that the only people he taught it to eventually fell to the dark side.
Four of the greatest Jedi and Sith fighting each-other at their peek of power. who would win? and why would they win?