Palpatine considered losing Maul in TPM to be a legitimate "loss", even though he immediately took Dooku(one of the most powerful Sith in the franchise) as his apprentice. For a point of reference, he didn't care about losing Dooku at all.
Based on that alone, it seems like Maul's potential was immense.
__________________
"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
Pre-bisected Maul had the potential to succeed PT Sidious:
All 3 of these sources say that Maul had enough potential to succeed Sidious, so it's pretty immense, but not as immense as Skywalker potential. Granted, one might argue that the third quote is from Maul's perspective if they wanted to be a smartass because of how it's phrased, but the first and third quote are not from Maul's perspective, they're the author's narration of the story, rendering Maul's in-universe perspective valid. Sidious himself has the same perspective:
Sidious has also said he can grasp Maul's potential for greatness in the dark side of the Force:
Only canon info I'm aware of that might be indicative of Maul's potential is from Son of Dathomir #3: https://i.imgur.com/UFOltIO.jpg
Talzin: "[Sidious] promised to make me his right hand, but instead he stole what was most dear to me [...] When Sidious realized Maul's potential, he took him from me and trained him as a Sith."
Depending on your interpretation of the dialogue, that could imply that Palpatine believed Maul's potential > Talzin's.
But again, it's ambiguous.
__________________
"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
Sidious chosed Maul as a potential heir to the Sith. This is supported by both Filoni and Witwer and also by that comic where Sheev said that Maul was a loss. Even if is not explicitly said, the authorial intent is clear. Sidious burned bridges with Talzin just to get Maul because he realised how big is Maul's potential. Sidious wouldn't invest so much in an aprentice whose potential is not at least comparable to his own. With Dooku is different as he was just a place holder or as Sheev said a proton torpedo.
it stands to reason that Maul had the potential to be Sidious' successor in canon because Sidious trained him with no knowledge of a Chosen One running around so he would've trained Maul to the best of his ability in order to keep the Sith line going and getting stronger and we know in canon that was Sidious goal based on how he lauds Vaders potential in episode 3
If you only use the beam struggle from the comics as evidence, then Palpatine ~ Talzin, and Dooku > Maul(in terms of power, at least.)
-Palpatine and Talzin seem about equal during the initial struggle.
-Maul then gives his power to Talzin, just as Dooku adds his lightning to Palpatine's barrage.
-Talzin is quickly overpowered to the point where she is barely managing to shield from the attack, and Force pushes Maul to safety before he is also destroyed.
Point being, if Maul and Dooku were close in power, then the combined beam-struggle wouldn't have been so one-sided.
Moreover, Mace believed that Dooku was the Sith Master, and Maul was his apprentice: https://i.imgur.com/JOm7JMT.jpg
...Which further lends to the notion that Dooku > Maul at the time.
__________________
"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."