The 45mm and the 110mm GFX lenses are both excellent, and reasonably fast aperture. The 250mm f/4 is amazing, but rather specialized, as is the 23mm f/4.
I end up mostly using the 23, 32-64, and the 100-200 for landscapes, and the 110 for portraits. The zooms are also very good.
Recommending one lens is challenging since we don't know what you shoot. For me, it would be the 110mm and I would shoot nothing but portraits. For you, it might be the 45 and you would shoot nothing but street and candid.