Which would it be?
Shotgun, Sniper or Fusion?
Bear in mind that some exotic secondary weapon types can be equipped in the Primary slot. e.g. Universal Remote and No Land Beyond.
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Fusion Easily. They're great at short to medium range, have high damage output, always come in an elemental type, and if you get some really lucky upgrades (or are lucky enough to get a purifier) they have some ridiculous combos to make them even a long range terror.