I think for me it would be killing Cabal. Have you seen what they look like under their helmets in the videos? They are uglier then Emperor Palpatine.
What enemy alien race do you want to obliterate first?
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I feel like it would be fun to sit atop a hill, pick off a Fallen pack one by one, staring with the highest in command and working down until the lowest rank remains, then emerge from my perch and engage in hand to hand combat.