Anakin Skywalker was one of the strongest Jedi in the films and Darth Vader was one of the best Sith. It only makes sense that they both could have decimated Darth Maul if they faced him. At the peak of Anakin's power, he was able to take on Count Dooku and win, and went toe-to-toe with Obi-Wan on Mustafar.
Maul and Grievous engage in single combat. The forces that landed engaged the Mandalorians in battle. Grievous sent a number of his droids against Maul and Death Watch, and Maul made quick work of those whom he personally fought.
Durge has a considerable edge over Grievous on account of his ability to regenerate. He was so resilient that he only perished by being thrown into lava.
Darth Maul VS General Grievous | BATTLE ARENA | Star Wars
Was Maul afraid of Vader?
Yep. Darth Maul didn't really say or show straight up that he "feared" Darth Vader, but he openly stated his belief that he could not defeat Darth Vader without the help of the Jedi, therefore on his own.
Though seemingly unbeatable, General Grievous famously met his end on Utapau, thanks to Obi-Wan, and was previously defeated by Mace Windu during the Battle of Coruscant, but Windu had also bested Grievous one other time, as shown in issue 5 of Star Wars: Obsession, by Haden Blackman and Brian Ching.
To say that Grievous would be able to overpower him within the first few minutes would completely neglect just how powerful Yoda is. In fact, he may be one of the few Jedi who could survive an encounter with Grievous and keep his lightsaber from becoming a part of the Separatist leader's collection.
Had they fought one another at their full potential, Maul's exceptional lightsaber skills would have been difficult for Dooku, but Dooku's knowledge of the dark side of the Force would likely have prevailed.
That, however, was not a coincidence, and there are two very good reasons for their on-screen separation. The first reason that Grievous and Anakin didn't fight in The Clone Wars was because the two of them couldn't have met due to a relatively unimportant line of dialogue in Star Wars: Revenge of the Sith.
The duel. Darth Vader defeats Darth Maul. The two Sith Lords engaged in a lightsaber duel, going through the volcano and onto a bridge situated over a river of lava, which collapsed, though they both survived and continued their battle.
So Maul vs Kylo. In terms of physical ability Maul is far physically superior being able to handle the strength of Savage Opress and overpower the strong style of Qui Gon Jinn. His durability involves being able to be cut in half and a pain tolerance so high he can flinch of weaker force lightening like nothing .
By the time of Episode 3; Anakin is in his prime, and already one of the Jedi Order's most powerful “masters” (although technically still a knight), just behind Obi-Wan, Windu, and Yoda. Maul wouldn't stand a chance.
Supreme Leader Snoke is presented in The Force Awakens and Last Jedi as an extremely powerful Force user. He is easily able to move objects with his mind from great distances and has other abilities that suggest he is more than capable of defeating his Apprentice, Kylo Ren.
He would use the seething rage pouring out of maul against him, and as mauls force power was so much weaker than windu's it would be an undisputed win for windu. They had a brief skirmirsh in Son of Dathomir (canon comic).
It all depends on how Ren approaches the fight. If he decides to make full use of his Force powers from the start, he crushes Greivous much as Windu did, just with less style. If he's dumb enough to engage in a fencing duel with Greivous he just might get sliced up before he realizes his mistake.
Mace does have some advantages, for example shatterpoint and Vaapad. However, even with these advantages Windu would be defeated by Vader. The Dark Lord is one of the best duelists in canon, slightly more superior than Palpatine to the point that even Palpatine indicated Vader's strength and power.
It's already been shown in the source material… more than once… that Darth Maul is not on Darth Sidious's level. Every time he dueled Sidious as his apprentice (in Legends) he got easily defeated. In the show Star Wars: The Clone Wars, Palpatine takes on Darth Maul and Savage Opress simultaneously.
Having trained in the ways of the Force, Darth Maul was a formidable warrior strong with the dark side and possessed deadly skills in lightsaber combat. After he lost his place as the Dark Lord of the Sith at Sidious' side, Maul remained a scheming mastermind who plotted to gain ever more power.
The most iconic battle in Star Wars: Episode I - The Phantom Menace is, by far, Obi-Wan Kenobi and Qui-Gon Jinn versus Darth Maul. When Obi-Wan finally managed to slice Maul in half and cast him into the depths of Naboo, he became the first known Jedi to have killed a Sith in a millennium.
