Use logic, feats, and stats.
Assume firepower and shields are equalized.
The First Order has StarKiller.
The Covenant have High Charity.
The galaxy's are merged. Covenant on one side, First Oder at the other.
Both at their peaks.
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First Order. They have stormtroopers who can actually hit stuff, Starkiller Base is way more powerful than High Charity, Kylo Ren could simply crush Covenant forces like tin cans with their own suits of armor, and the First Order could destroy the halos with Starkiller even if the Covenant found them. These are all valid points. But lets face it, First Order wins because of one man, FN-2199, also known as.... TR-8R.