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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I'm going with the Covenant on this one. We don't know the exact size of the first order, and judging by the small X-wing strike force that was able to destroy starkiller base, (with help from lowering the shields from the surface) I believe the covenant would be able to to take starkiller base within time. As for ground battles the Covenant would stomp due to the extensive array of infantry. The first order did have ground vehicles but we never saw them in action.