Fastest in terms of how fast it can fire or in terms of how fast the bullet travels?
In the case of RoF, the fastest would likely be a rocket launcher with 500%+ Burst Fire. This would effectively fire 5 rockets one after the other. Helix's are similar and do this fairly well. They fire 3 rockets in a "helix" (circling each other). Helix's can come w/ x4 elemental effects.