IMHO Tachyon Cannons are the best weapons in Privateer... but for good shooters only. You can buy Plasma but it's "one shot only" (good for newbies with aiming problems; like I was many years ago) because of energy drain.

In "Righteous Fire" extension player can buy Fusion Guns and then TCs are "too old"... ;-)

I'd stick with Tachyon Cannons. They have a good fire rate and power drain (about the same as Meson Blasters) and better damage than those. Plasma Guns have the best damage, but slowest bolt speed and slowest fire rate.

I'll put in another vote for Tachyon Cannons. They have good damage, and the damage/energy ratio is the best out of all of the weapons, meaning you can easily spam 4 cannons simultaneously without putting too much of a strain on your energy supply. They also have one of the highest velocities of any of the weapons, as well as one of the highest rates of fire. Overall a very good combination that no other weapon can really stand up to.