Last May, the Fujifilm GF 50mm f/3.5 was a 'steal' at $500 off (50% off), so I bought it even though I did not have a camera body to go with it, suspecting that a camera like the Fujifilm GFX100S might come along within a year.
Sure enough: a much smaller and lighter camera body, and a lens that is truly svelte for medium format and small even for 35mm.
The 50/3.5 is IMO not only one of the finest lenses for the Fujifilm medium format system in terms of optics, it is also the smallest and lightest and most appealing to carry and the least expensive too.
On top of that, the 50/3.5 was-is my favorite all-arounder lens on medium format (any brand), suitable for a wide variety of uses including stitched panoramas* and focus stacking, in part because of its ultra-low distortion. And it’s darn good for astrophotography too.
The 50/3.5 is also an ideal lens for general travel photography, 'street' photography, environmental portraiture, group portraits, and even architecture.
Check out the lens hood too—it contributes to a compact shooting experience that still works with a polarizer.
Keep it simple, keep it enjoyable
That’s a lot of 'mileage' out of a single inexpensive lens.
* In portrait orientation, the 50mm focal length (41mm equivalent) is ideal for many panoramas. It is also eminently useful in landscape orientation. See for example, First Sunlight after Bitterly Cold Night.
Important to my B&H loaner support — buy your Fujifilm GFX100and Sony A1 and lenses via links on this site.