18. Tractor Cannon
If Bungie's aim was to make an exotic weapon perfect for trolling then they certainly hit their mark. However, if they were trying to go for something useful they missed by a mile.
The Tractor Cannon is a "shotgun" that essentially just gets enemies away from you. It doesn't shoot a slug or anything like a traditional shotgun, but instead shoots an impulse that pushes enemies back. It's exotic perks allow you increased movement speed after dealing damage with it, so it's good to use to create some space between you and enemies. However, it doesn't do high damage, which makes it almost useless in the power weapon slot unless you need an escape plan and don't mind losing out on a power weapon to have one.
About the only use this gun has is to troll in the Crucible. If you're looking to get on everyone's bad side, then go ahead and take it in there and try your best to push people over ledges with it.