For me, the FAS and the Python have been my go-to RES kings. FAS has 1x large beam laser and 3x PAs; Python has 2x large beam lasers, 2x multi-cannons, and 1x large PA.
You can make the ammunition last a surprisingly long time if you play your cards right. I've gone 2 hours straight in a High RES without needing to reload, and only went back to the station to cash in my bounties and take a bathroom break.