Thank you for making this topic for me.
Anyway, I felt we should try our best to establish the companions due to the fact Revan's perhaps most impressive combat showing has them included. I'll go through the list of companions and post my thoughts.
"The next hour is spent frantically blasting, swiping, and crushing countless Dark Jedi and Sith heavy weapons and elite troopers on your way through the decks of the space station."
―Star Wars Knights of the Old Republic Prima Guide
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Carth Onasi: Carth was always displayed in Knights of the Old Republic as your second leading companion, in my opinion (first if you have a female character). It is ultimately him and Bastila Shan who are captured on the Leviathan and confront Admiral Karath and Darth Malak there. Though, to be honest, how likely is it that a Republic Solider, regardless of skill, is capable of surviving the entire Star Forge run? Like I don't care if he hid behind Revan the whole time, I have no clue how he would be able to survive. There is also the fact that, in the game, your companions do not speak to Bastila Shan when you confront her. Carth, given his history with Bastila being the largest here besides Revan, and that he talked to her if you choose to become the Dark Lord of the Sith on the Unknown World, I find it unlikely he would just stand there and do nothing. I am going to rule out Carth being a companion until further notice.
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Mission & Zaalbar: There is honestly know way that Revan would have took a a 14 year old girl who is not even specialized in combat to begin with, and if he would have took her, he would have had to have taken Zaalbar as well. While a Wookiee might prove useful, neither would realistically make it far on the Star Forge, especially not Mission. The fact all his companions survived makes me also say with confidence that neither of these two went with Revan.
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Juhani: Ah, the 'Cat lady'. ****ing hate her. Anyway, Revan most likely did not take Juhani. Above I colored the fact that Revan did indeed take a non-Jedi on the Star Forge, and to be completely honest, if you are going to take a Jedi onto a deadly Dark Side nexus, why take Juhani and not Jolee? Jolee has walked along the Dark side and been able to see the light. Meanwhile, Juhani, since you met her, has constantly been in conflict over the light and dark and which side to choose. Taking Juhani onto a a Dark Side nexus that, before the events of The Old Republic and Knights of the Old Republic II were created, were apparently the reason why Revan and Malak fell to the Dark Side, would be honestly stupid.
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Jolee Bindo: "The crazy old man who lived in the dangerous woods" was indeed pretty badass. If one is under the assumption that Revan took a Jedi companion on his gauntlet up the Star Forge, Jolee is the obvious choice. He is a very nice experience with the Dark side and conquering it, making him the ideal character for guidance over any temptation Revan must encounter on the Star Forge. However despite this, I am not positive Revan took him. Though extra support is always nice, the fate of the entire galaxy has more importance. If Revan would have failed to have defeat Bastila Shan and Darth Malak, the Republic armada would have been utterly crushed, and the Sith Empire would roll over any resistance left of the Jedi. It is a much smarter decision to leave Jolee on the Ebon Hawk in case something happens, as backup ya know? I won't state Jolee definitely didn't go, though I feel other companions are more likely.
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T3-M4: I am not going to rule out the possibility that T3-M4 went with him due to the fact Revan was confident enough to take him into the Unknown Regions against an 'unknown evil', though I just find it unlikely. T3 is not specialized in combat, but rather a maintenance droid who is a master at slicing. And yes while he was upgraded and modified to perform combat, why would Revan take T3 along to confront Darth Malak himself when he has more deadly companions? It just doesn't make sense, to be honest. Though like I said, it is still a possibility. After all, the Hero of Tython took his droid to confront the Sith Emperor.
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HK-47: I find it likely actually that HK-47 accompanied Revan on the Star Forge. He was specifically created by Revan for the purpose of assassination, and has killed a large amount of Jedi who Revan thought of as a threat throughout his career. In fact, Malak even thought of HK-47 so highly in combat he was confident that he could kill Meetra Surik after the battle of Malachor V and was even described as "one of the most dangerous assassins in the galaxy". Later he was even able to hold his own against the Sith Strike Team, and though he used the Foundry to his advantage, at some points were one-on-one combat. Out of all of Revan's companions, he had been with HK-47 for the longest by far, even if he doesn't remember it. Out of all the companions, I feel HK-47 is nearly a definite he went with Revan, to be honest.
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Canderous Ordo: Mandalore truly was Revan's "man until the end." The Revan novel further implies he went with Revan on the Star Forge with the dialogue exchange at the bar. Though I will motion to the Dark Side version of the game. Take notice that under normal settings, only Bastila Shan, HK-47, T3-M4, and Canderous Ordo accompany Revan off of the Unknown World. Zaalbar can as well but it requires some mind-domination. Canderous is not slain like the others, which is a subtle reminder of his importance over some of the other companions. Being a Mandalorian who has killed many Jedi in war, Canderous, like HK-47, seem to be the right choice to go when thinking of what companions Revan brought.
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Final Thoughts: So, Canderous Ordo and HK-47 seem to be the most likely, though Jolee Bindo and T3-M4 are still a possibility.