I think that we have more or less agreed that quad Tachyon cannons frag things the easiest. Mass Drivers seem to be the next most popular--they deliver a good punch with low power consumption--their main drawback being the short range.
I would like to put in a word in favor of the plain vanilla Laser cannons, which seem to be an underdog here. True, they have the lowest damage potential of any gun, but, prior to the introduction of the Dust Cannon, the Pulse Particle Cannon, and the Chain Ion Cannon in SOP, they were the fastest-firing guns available. Also, they have the highest range in the game, nearly 50% farther than the Ion, Tachyon, and Particle cannons and their variants.
The low damage potential of Lasers can be offset by just having more of them--for example, the Banshee in WC4 had four Lasers, and was a good light fighter--the Lasers were enough to fight any opponent except for Dragons, which you were better off using the Leech cannons against. In fact, I have modded a Tigershark variant that sports six Laser cannons, and it seems to work well for dogfighting at long range.