It also depends on your difficulty level. On higher levels you'll find that energy drains much faster, so your double Kayser combo won't take you as far as a Kayser and a Subach will.
Generally, though, the Kayser is really powerful but with short range and high energy usage; the Prom S is a good, well balanced weapon; and the Subach is a light but decent weapon if you want to divert energy to shields or engines. Stay away from the Prometheus R, it's not worth it.
The rest of the cannons are best for tactical use: Maxims are perfect for picking off turrets, the Morning Star is a good bomb/bomber interceptor for slower fighters, and giving your covering fighters a Lamprey occasionally will keep the enemy off your back.