Based on the lenses I have, for the digital bodies the DA 35mm ltd.
For AF film the FA 50mm f1.4 and for other k film bodies like the Super Program and K2 the A 50mm f1.2.
For m42 it would be tough between the macro-tak 50mm f4 and the Auto Tak 55mm 'Zebra'
However, the reason I originally got into interchangeable lens cameras was to not get into the just 1 lens situation.