FF, RAW, and Live View. Put those together with TS-E lenses and Photoshop stitching and you can compete on image quality with the large-format shooters—which is astonishing.
Nothing there that I genuinely couldn't live without but if I could only have one from the list it would have to be RAW. Shooting jpeg would seriously alter my workflow.
I voted FF because I tried to go back and couldn't lol. RAW is a huge asset to my photography but have to squint into an APS-C viewfinder after having a D700 for a year made me want to leave my camera at home all the time. I just came back to Canon with a 5Dc and am once again thrilled to be shooting...ANYTHING.