The almighty is irrelevant in this discussion, as the Red Legion have no way of knowing which system to use it in to maximize its threat to the Covenant (a group of multiple species from different planets, possibly and very likely in different systems) whereas with Earth, they already knew since the Cabal had been on Mars already. Also, there is [b]no way in hell[/b] the Red Legion has [i]more[/i] ships, or even [i]better[/i] ships than the Covenant. Unless new canon info is released about Cabal ships, there is nothing to prove they can output more destructive potential than ships that can glass planets and whose plasma torpedoes can melt through multiple layers of Titanium-A ship plating (I'm ignoring Leviathan because it's not [u]controlled[/u] by the Red Legion, and I'm only focusing on them, as per the topic question). Also, let's see how well the Red Legion fare against scarabs. tl;dr Covenant stomp very easily. Damn, now I want to go back and read my Halo novels.

I'd go for the Cabal in general. Both groups are space empires who conquer and subjugate other races and are known to blow up/glass planets/solar systems for various reasons after first assaulting and destroying various ground and fleet based infrastructure with seemingly endless amounts of troops and resources. It's actually a bit difficult to compare who has more soldiers/ships/resources what have you as both are rather vague about that kind of logistical sense and the actual answer for those kind of questions is in reality however many there needs to be to tell the story. However there are some key differences between the two that puts favor towards the Red Legion. First and foremost, technology. The Covenant are specifically noted to imitative in their technology rather than innovative. Everything they create is based off a forerunner design and they do not make any attempts to understand or improve said technology as it goes against their religious teachings and ultimately their technology comes down to swords, shields, ships, and guns. If I recall correctly the UNSC is more advanced then the Covenant at the end of the war. In a direct comparison the Cabal are noted in game for being able to analyse and comprehend Vex tech, have sensors on every soldier to completely analyse the battlefield in real time, are able to analyse and reverse engineer Hive magic into a techno-logic base in order to capture the Traveller, [spoiler]Do the same to the Taken in order to steal light from the Traveller and empower Ghaul, and are implied to have built both larger then planet sized ships on their own.[/spoiler] That they make some of these improvements during the game implies an active use of science and engineering to improve their capacity that the Covenant simply do not have, and would be a major advantage in any conflict between the two. The other big advantage that the Cabal have is physiology. The main force of the covenant by pure numbers, if I recall correctly, are the Grunts. While the main force of the Cabal are...well, the Cabal. They are just stronger, tougher, and have less tendency to panic in general. I can't really imagine a scenario in which the Grunts could defeat Legionaries that did not involve the Grunts outnumbering the Cabal by at least an order of magnitude. While Elites and Brutes are probably of similar height to an average Legionary, the Legionaries seem, to me at least, to have more mass in general which would suggest more strength and endurance in general. Hunters certainly outclass legionaries in terms of power but that's rather easily matched by the various Colossi and Valus the Cabal have, who are all combat veterans in their own rights. The Cabal just in general more physically powerful. Then you have to take into consideration the Psions, who fill roles similar to the Jackals, Drones, and Engineers, are probably all around better at said roles do to the fact that they have psychic powers with the strongest of Psions capable of altering the orbits of heavenly bodies (Psion Flayers). I'm sure there's other things I'm forgetting but those two points certainly point to an overall Red Legion advantage.

The Red Legion are a miniscule force compared to the Covenant. The Covenant has them beat by ship numbers alone. This is like asking "who would win, the 1st Infantry Division or the entire Chinese military?". It doesn't matter that 1ID is better trained and equipped, they simply don't have the numbers or weapons necessary to fight off an endless horde of similarly-but-not-quite-as-well-equipped-and-trained soldiers and other military millieu

It depends. Do the covenant have the Arbiter from Halo 2? Cuz he can wreck shit.

I just thought of a pretty great comparison in terms of strength for the Cabal and Covenant. Thresher gunships take, on average, 3 Drake shells to destroy, where-as a Covenant Phantom has regularly required at least 5 Scorpion shells to destroy. This either means the Thresher has significantly weaker armor, or the Drake is significantly more powerful than a Scorpion. Either way, it doesn't paint a good picture for the Red Legion versus the Covenant.

Ground wise....Covies. Ship to ship engagement....covies. Superweapons....Cabal It's really circumstantial. If the Cabal have enough time to deploy The Almighty, successfully defend it, and fire... maybe Cabal win. Unless the supernova disrupts the Covies ability to slipspace out of dodge... they'd just warp out. Though it only took one Guardian to stealthily neutralize The Almighty.... which Sangheili and Jiralhanae (probably butchered the spelling) are capable of. However if the Fleet (of Particular Justice) that took out Reach is present.... that would hasten the Cabal to nuke the hell out of the system. Without The Almighty present... I go with Covies.....do to sheer numbers. Though..we don't know too much about Cabal Ship shielding and Armor.... too make a truly accurate victor.

Halo 2 Jackal snipers. Nough' said.

Shit... I mean the Covenant glass planets, but with an entire fleet next to the Almighty, would they ever get the chance? Then again, both have teleportation tech.