The answer really depends on what kind of enemy is being fought.
For smaller, swarming enemies (eg.Cragpoles, Tesladrones, etc.), the Shock Ravager/Lightning Ravager works best, with the Pyro Blaster in second place.
For larger enemies , which attack in smaller groups, (eg.Drophyd Troopers and Sentries, etc.), the Predator Launcher/Raptor Launcher works well, with the next best being the Nano-Swarmers.
For the kinds of tough enemies which attack by themselves (eg.Cragmites) and Bosses (eg.Crushto and Zorthon), the best weapons to use are the Negotiator/Judicator and the Alpha Disruptor/Alpha Cannon.
Of course, the best weapon to use to get the most firepower (for attacking large swarms of strong enemies, or for bosses) is the RYNO IV