I prefer to mount the dual Parabellums in the German 2-seaters and jump into the back seat more often...I've found that the shredding effect at close range to be eminently satisfying when a fighter tries to get on your tail.

So far I've tried all the weapons mods including the rockets ( can't quite figure out how to hit anything with those ) and I have to say, my favorite is by far the twin Parabellums...however, the Bristol's twin Lewis guns in conjunction with her single Vickers or the twin Lewis's on the DH.4 with its twin Vickers are formidable to say the least.

The Becker cannon on the Albatros D.II or in the backseat of a German 2-seater is quite lethal, but seems difficult to get hits at anything other than almost point-blank, so I tend to not equip it for missions.
All in all, I would say my habits have changed considerably since the weapons mods became available, as the Albatros D.III with its trophy overwing Lewis has become my new favorite to hunt RE.8's with.