Obi-Wan originally used Form IV commonly known as Ataru like his master,
Obi-Wan Kenobi was highly skilled in Form III. Over the course of the Clone Wars, Kenobi continued to develop his skills.
Throughout his career as a Jedi knight, Obi-Wan used Form III Soresu in lightsaber combat, which was originally created as a counter to the dueling intricacies of Form II's fencing style and focused on minimizing movements while optimizing defense.
He has used multiple Forms to improve his prowess as a duelist since the start of the Prequel Series. By the time of RotS, Obi-Wan exercised Forms 1, 3, 4, 5 (Shien) & 6 in battle, Soresu and Ataru being his main focus in their stated order.
Practitioners of Form IV included Yoda, Qui-Gon Jinn, Aayla Secura, Ezra Bridger, the Grand Inquisitor, and Anakin Skywalker.
In more recent years, the form was mastered by the Jedi swordmaster Cin Drallig along with the other six forms. Grand Master Yoda and Swordmaster Mace Windu were also proficient in its usage, as they were said to have fully mastered all the seven forms of lightsaber combat.
Practitioners of Form V included Jedi Masters Aayla Secura, Adi Gallia, Plo Koon and Luke Skywalker, Jedi Knights Anakin Skywalker and Ulic Qel-Droma, and Dark Lord of the Sith Darth Bane.
Jedi Battlemaster Cin Drallig and Grand Master Yoda had knowledge and mastery of all seven forms, including Juyo, befitting Drallig's role as lightsaber combat instructor at the Jedi Temple and Yoda's reputation of having mastered all forms of lightsaber combat as did Anoon Bondara, another lightsaber instructor.
Form VII, known by its two primary disciplines of Juyo and Vaapad; and also known as The Ferocity Form, was the seventh form of lightsaber combat. It was considered the most aggressive and unpredictable form.
Form II relied on careful and controlled strikes instead of power and strength. Count Dooku was a master of this form and trained the Sith assassin Asajj Ventress in its use, though hers was less elegant than her master's.
They typically taught the form to Jedi Sentinels. During the New Sith Wars, Niman was practiced amongst both the Jedi and the Sith forces. A notable Jedi adherent was Johun Othone, who took full advantage of Form VI's inherent versatility during the pitched battles of the war.
A prime example is Kit Fisto. Kit Fisto was a Skilled Jedi of form I who fought during the clone wars. He almost defeated general Grievous using Form I of lightsaber combats. Other than Fisto, Meera Sutrik, and Obi-Wan Kenobi are the notable names to use Form I of lightsaber combats.
Form VII: Juyo (or Vaapad)
Form VII is the last known form of lightsaber combat and is known to be the most aggressive. Jedi Master Mace windu is the main Jedi known to use this form of combat because Form VII is the most aggressive and only the most Force-sensitive individuals can execute this form successfully.
Jarrus later used Form III against the Grand Inquisitor, who recognized it as a staple of Billaba's instruction and was able to counter Jarrus' attempts to use it against him. Obi-Wan Kenobi also mastered the Soresu form of combat.
Kenobi is known to be a great master of Form III. He used this form to beat Vader in Mustafar. However, I believe Obi-Wan was also brought up with Form IV and he used this to beat Maul on Naboo. He only switched styles after witnessing Qui-Gon's death.
Kylo Ren's lightsaber form was something of a hybrid, which blended three styles together: Djem So, Juyo, and Niman.
Form "Zero", while not an actual form of lightsaber combat per se but rather an ethical principle, was the idea that Jedi should know when to use their lightsaber and when to find an alternative means of solving a problem.
Form IX: Kadeesh was a Form of lightsaber combat mainly focusing on the use of brute strength rather than Force-assisted movements or fancy and unpredictable parries.
Form VIII: Kendosu, or the Way of the Varactyl, was a new swordplay style developed during the Great Galactic War by Jedi Knight Scott Borek.
Audiences didn't see Yoda wield a lightsaber until his duel with Count Dooku in Attack of the Clones, but Yoda was already more than proficient in lightsaber combat. In fact, Yoda was trained in all seven forms of lightsaber combat and had full mastery and knowledge of all of them.
During his years as a Jedi, Obi-Wan Kenobi fought with three lightsabers. His first had a blue plasma blade and a hilt similar to that of his master, Qui-Gon Jinn. Obi-Wan used this weapon until Darth Maul kicked it into a deep shaft on Naboo.
Billaba became Windu's Padawan and trained under the Jedi Master in the ways of the Force. During her training, Windu allowed her to study Form VII variant known as Vaapad. She eventually became a Jedi Knight, renowned for her skills with a lightsaber and favored Form III when teaching other Jedi.
At first, Rey primarily fights with a lightsaber the same way she would have with her quarterstaff. However, after studying Luke's Jedi scripture and training with Leia, Rey probably uses Form VI: Niman, a jack-of-all-trades lightsaber form that combines different aspects of Forms I through V.
You might not expect it from his calm demenour, but when you consider Qui-Gon's backstory, you'll understand his chosen form: that of Form IV, Ataru, or 'The Aggression Form'. Qui-gon is a rebel, constantly going against the wishes of the council in order to carve out his own path.
One of the reasons Anakin was such a good duelist was the combination of his Force powers and his chosen lightsaber style -- Form V, Djem So. It was a form associated with power, aggression and counter-strikes.