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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The first order doesn't have a fleet the size of the covenant but Starkiller would be a problem. If the covenant rushes the First Order fast and take out Starkiller before it destroys high charity they would win