In Legends he matched Windu briefly in a duel, and he had a computer system in his brain that copied Lightsaber Styles from his opponents, which means he had partially copied Windu's Vaapad, if you give him the Force along with how OP he was, he would be definetly above Dooku, Anakin, Obi-Wan and Windu.
After accepting Dooku's invitation, Grievous sacrificed practically all of his physical body and nerve endings to increase his combat abilities and gain the strength and speed to rival even a Jedi Master through cybernetics. He became an even more threatening and fearsome warrior.
It was not until Chapter 25 (or Season 3 Episode 5 if you prefer) that Grievous had a showdown with Mace Windu. During the encounter, Windu used the Force to crush the chest plates of Grievous, severely injuring his lungs, and causing his now distinctive cough, which made its way into Revenge of the Sith.
Grievous was trained and designed to engage with Jedi in a way that would disadvantage them by exploiting their ways. “A Jedi uses the Force for knowledge and defense, never for attack.” The Jedi Generals in the Clone Wars were trained and raised to never use the force for attack.
Grievous utterly hates the Jedi and their evil Order for committing genocide on his people when he was a warlord. He exploited the Jedi's "compassion" for the clone troopers under their command, exemplified during his attack on the Kaliida Shoals Medical Center.
Sidious wasn't scared of Grievous as he served him as a means to an end, rather that he was concerned about what the Kaleesh Cyborg would do if he ever knew about Sidious orchestrating both sides during The Clone Wars.
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.
There is also an in-universe reason that Grievous and Anakin never met each other. Anakin was instrumental in Darth Sidious' plan to overthrow the Republic and the Jedi. He needed Anakin for his machinations, but he also needed General Grievous to keeping razing the galaxy. He couldn't afford to lose either of them.
Basically, Palpatine knew that if Anakin and Grievous ever fought, it could mean one of their deaths, probably Grievouses. So he kept them apart so that they could each play. their roles until the time was right.
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.
They hold that Obi-Wan's defensive style of fighting confounded Grievous. The villain defeated many Jedi by turning their own styles against them, but Kenobi played the long game, protected himself and waited for an opportunity.
But he did have one glaring weakness. grievous was not Force sensitive, meaning he could not put up a Force Wall to defend. against Force attacks, so Grievous was always at the. mercy of being thrown around with the Force.
Maul and Grievous met in single combat, while the two armies collided and suffered heavy losses. By the battle's end, Maul ordered his forces to retreat, while the Sith waited for him to find Mother Talzin.
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.
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.
While the Jedi would remain his sworn enemy, Grievous was a threat to Force users outside their order as well. In fact, perhaps his greatest kill was not a Jedi at all, but a dark side adherent of such immense power that even Emperor Palpatine feared her as his equal.
Grievous was originally Qymaen jai Sheelal, an organic reptilian humanoid native from the planet Kalee. The Kaleesh were at war with the rival Huk species and Sheelal was considered a demigod by his people for his victories in the Huk War.
