The Battle of the Pelennor Fields was only a side-light of the War of the Ring; so it was with every battle in the War of the Ring, save one: the battle for Frodo's will as he bore the Ring to Orodruin. Had Frodo succumbed to its power at any time before he did, the West would have been lost into millenia of darkness. That's huge. So that's my vote for the 3rd Age.
For the 1st Age, the West versus the North as has been said.
For the 2nd Age, the Battle in which Sauron loses the Ring.
Of all of these, the 1st Age battle probably takes precedence, since its results directly affected the later ones.
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