Which lightsaber form did Obi-Wan use against Maul on Tatooine?

Yes, Kenobi used Ataru

This is an interesting question. In part because it's difficult to read much into a three-strike duel.

To break down the duel, as you mention in the question, Kenobi starts with a Soresu stance and shifts to Ataru (presumably to trick Maul). One of the characteristics of Ataru is broad, sweeping, strikes. And Kenobi's first strike/parry is definitely this as he brings the saber in a full 360 degree rotation to block Maul's strike to his head.

However, Kenobi's second strike is more in keeping with a Soresu defense. You can see that he takes a small hop backwards and brings his saber down to block with the smallest amount of movement necessary. Minimization of motion is one of the characteristics of Soresu, and is in keeping with Kenobi's prefered style.

Finally, the third, and killing blow was another full circle strike from around behind Kenobi's head, through Maul's saber and torso, and following through back to nearly the original position. The sweeping nature of the blow is Ataru, but the choice of action is very much Soresu (capitalizing instantly on mistakes).

We could discuss this duel until the cows come home, talking about motivations and the like, but I think I would break down the fight as follows.

Kenobi opens with Ataru, his stance almost mandates this. Additionally, he chooses to meet the strength of Maul's blow, so for that reason, the sweeping blow was the one he used. His second strike was instinctive, as evidenced by the small hop backwards that is universal for, "I need to move right now without thinking about it", as well as being in the Soresu style. Had he continued focusing in Ataru after Naboo, this instinct would have almost certainly been a flip over Maul's saber. The third and final strike returns to Ataru, his choice for the duel (or at least the beginning), to bring an overhand blow down on Maul.

Ultimately, what we see is a true dueling master. One able to hybridize the forms and adjust as necessary. The second strike betrays Kenobi's love of Soresu, but I think the final answer is that Kenobi chose to open the dual in Ataru, and ended the fight before needing to reevaluate.

To answer the question posed in your comment above: Yes, it is still Ataru without the acrobatics. "Another characteristic of the form was it's fast, powerful strikes from multiple directions", which Kenobi did utilize.

He used both. Soresu was his main form for sure but if the situation called for it, he would use Ataru for some of his more advanced attacks. Obi-Wan is no stranger to mixing multiple Forms in combat in terms of both practical applications and their principles. 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. While a Padawan, he placed his Form focus on 1 & 4 with some technical additions from Shien. Jinn was the reason for Obi-Wan focusing on 1 & 4 but unlike Qui-Gon, Obi-Wan wasn't as advanced with Form 4 so he couldn't use it in a more advanced way to deflect blaster fire with Form 4 like his master. So he added some Form 5 influences to improve his blast deflection skills. You could tell from the way they both deflected blaster bolts in the opening battle against those droids. Kenobi's deflection skills were keen and quick while Jinn's deflections were broad with more kinetic motions behind them.

We all know that Jinn's death is what made Obi-Wan give up Ataru as his main form and take up Soresu. By the time of AotC, he had pretty much mastered Soresu for blast deflection but it wasn't on par where he could use it for saber duels against veterans like Dooku. You could tell from the fact that Obi-Wan placed more emphasis on using Ataru against Dooku. He was confident with Form 3 against blasters no questions asked but against sabers, he wasn't ready. Obi-Wan wanted to make a Hybrid form of Form 3 & 4 after TPM but since he hadn't mastered either of them to their fullest yet, it didn't work out at that time. It's what made him lose out to Dooku so quickly. That and since Dooku pretty much was an expert at destroying Ataru users, the whole thing was just stacked against the poor guy.

He'd come to master Soresu to the fullest throughout the clone wars and even study Form 6 to allow him to improve his overall application of the Force during Saber combat, as well as learn to dual wield. The greatest benefit he got from Form 6 though is it allowed him to hybridize his styles far more effectively than before as he had intended. Form 6 is a jack of all trades type of style. Thus, this allows him to mix styles without issue. Even styles that completely contrast each other could be mixed in without problems. For example, Soresu could be combined with Juyo, two styles which are COMPLETE opposites of each other in both practical application and principle.

Come RotS, he had completely mastered Soresu for combat and as such, Dooku couldn't even pressure him with the Saber anymore, which is why he had to use the Force to even get Kenobi off of him. But the duel with Dooku aside, Kenobi being able to use multiple forms should be a no brainer at this point. The only Jedi alive in Kenobi's era capable of using more than 3 different Forms on an expert to master level is Yoda. Every other Jedi has mastered 1 Form with a complementary form.

As for the discussions regarding the use of Makashi and Djem So, Kenobi never touched either of those styles in his life. While he could add Makashi to his repertoire, it wouldn't fit him. First, Obi-Wan placed heavy emphasis on defence, Makashi, despite being Soresu's direct predecessor and sharing some key traits with it such as the economy of motion, didn't fit his fighting style. Soresu's bladework generates a good degree of kinetic energy through its circular motions which allow Obi-Wan to use that kinetic energy for powerful counterattacks, Ataru being able to make full use of this built up energy. Makashi doesn't generate kinetic energy at all and while its core principle is shared with Soresu, their approach to that goal is completely different. Their foot techniques are also completely different and as such, they will rarely mesh in that regard.

As for Djem So, it won't suit Obi-Wan at all. First, his body isn't built for it nor is it compatible. Obi-Wan isn't a large man. Compared to most of the characters shown in the series, he's height is pretty average. Look at Anakin, he's way taller than his master, he was literally born to wield that form. Not to mention Djem So requires the wielder to be physically strong to boot as well. Kenobi placed emphasis on styles that allowed him to fight for long periods of time in all sorts of scenarios. Djem So would kinda kill that concept. He could learn the first of Form 5: Shien since it placed more emphasis on speed and blast deflection and it probably aided him in mastering Soresu so quickly after all of his years of focusing on Ataru. But Djem So? Nah, he wouldn't use it.

Just in case people are gunna ask what about Form 7, this is a no brainer. Form 7 in general was forbidden for public training and only select individuals were given permission to train and use it. In the case of Vaapad made by Windu, Jinn forbade Obi-Wan from learning it since it is even harder to learn.

This duel seems different than either of these styles. Remember that Obi Wan has been waiting in the desert for close to two decades, his only purpose is to protect and guide Luke. During that time he may have mastered a few more styles, and even created his own versions of a few of the styles. While it doesn't look like Dooku's use of it, it sounds more like the base descriptions of Makashi. Either that or the Djem So variant of Form V.