Kojol and Tridax are more of background Makuta throughout the story, but I think significant evidence suggests that they were actually the second most powerful members of the Brotherhood, even more so than Antroz and Icarax.
First off, the resumes of Kojol and Tridax blow all of the other Makuta out of the water besides maybe Mutran and Icarax, and I think their accomplishments hold the edge over even those two. Kojol created the virus that put Mata Nui in a coma and was responsible for the raid that stole the Avohkii. Tridax possessed a Kanohi Olmak, probably the most powerful non Legendary mask in the universe, and created an army of shadow Takanuvas for the Brotherhood. He also designed the shadow leech pods all the other Makuta wear.
On Kojol
In The Mutran Chronicles, Mutran regards Kojol as arrogant and incompetent, and does not warn him or Teridax that the Order is planning to assassinate him. This actually supports the idea that he was extremely powerful - Makuta are jealous of anyone they perceive to be above them, as shown by their hatred of Mata Nui. Mutran also has a feud with Chirox, but he regards Chirox with a sort of disdainful contempt, and decides to spare his life when he has the opportunity to kill him during the evolution to rub his new Rahi in his face. Compare that with his genuine venom towards Kojol, and the fact that he was so eager for Kojol to die he didn’t even bother to get the location of Artakha’s island from him.
Mutran believes that he was responsible for Kojol’s death by not warning Teridax, but there is zero chance someone as smart as Teridax didn’t notice everyone involved in the Artakha raid getting killed. Kojol, who was responsible for felling Mata Nui, was the single biggest threat towards his end goal of becoming the Great Spirit, and since he already stole the one thing Teridax needed from Artakha, the Avohkii, Teridax was perfectly content with allowing Kojol to be killed. Even thousands of years after his death, Kojol was apparently notable enough that The Shadowed One and Ancient immediately recognized his name and the circumstances of his death.
On Tridax
Takanuva, as a Toa of Light, is canonically the biggest combat counter to a Makuta in the universe. Even Mutran admits that the power of a Toa of Light is “the one power we Makuta dreaded”. Despite this, in order to build his Shadow Takanuva army, Tridax clearly defeated and subdued hundreds of alternate Takanuva. Tridax is usually assumed to be a less powerful Makuta since he wasn’t invited to Karda Nui, but the fact that he was undefeated against so many Toa of Light suggests that this probably wasn’t the case. Tridax was actually kept at Destral because Teridax would not want the Olmak to be incinerated in the energy storm, since a Shadow Takanuva army would be much more effective than a Rahkshi army in his space domination schemes. Teridax was probably actually not expecting Tridax to die before the shadow Toa army was done, but since he was already the Great Spirit by the time he found out, he was fine with the minor loss.