That is a very subjective question. It completely depends on your play style. However the weapons that get the most attention are these two:
Maliwan Hellfire Maliwan Volcano
They both are fire elemntal weapons with x4 modifiers and a guarantee to ignite the target. They also have a chance to cause a larger explosion which will burn other targets around the shot. I feel that the Torgue Cobra is similar to if not better than the Volcano yet it gets very little attention.