Another good combination is the Thorin 300 multicannon + Rasiermesser Weapon Enchancer (75%) + Lithium Booster (50%)
I like my Dvalin Ion9 cannon in my Helgoland A110. I becomes omnidiretional.
Kourtious' Equipment and Ship Guide
Just to be obnoxious - since I usually am playing with a crapton of mods, my favorite endgame weapon is the Ares Plasma Archcannon. Takes 2 slots, needs the Hyperion reactor, but once you have that you one-shot anything before the last few systems. Only problem is that you need to farm Ares Dreadnoughts...