Currently the FAS.
I spent the first two months of Elite doing non-combat work, gaining rank and making some money in my Cobra Mk3 and an AspEx. I love the Cobra - feels like an old car I'd never get rid of.
Three weeks ago I wanted to start gaining combat experience so I bought a Vulture and started taking it to RES sites. Mostly went well except for the two times when I became wanted - once for shooting a cop by accident, another time when I was rushing in to take some shots on a Python the police were killing, and failed to complete the wanted scan. Once, the cops killed me. The other time I managed to just barely escape.
Anyway, the Vulture is fun and all, but I wanted to step into something bigger, so I read about the FAS and took the plunge the other day. Mostly A-rated it, took it to the RES, and in my first fight with a FGS lost all sense of situational awareness, forgot to kill the fighter he launched along with his wingman before killing the FGS. My shields quickly evaporated (they were biweaves at the time), hull started taking damage, and while I eventually prevailed, I was at 26% hull. I headed back to base, repaired, and switched back to the Vulture for two days. I needed to make back some of the money I spent on the FAS.
Last night, I got back into the FAS, changed the shields to A-rated, put on two shield boosters, and went back to RES. I also changed tactics slightly. Mostly FA-off and not afraid to ram shieldless enemies, I was on a tear. Two hours and 6 million credits later, I'm in love with this ship.