One lens?

For a zoom:

Maybe my old 28-70 f/2.8 Nikkor. Fine on both DX and FX cameras, tack (prime lens) sharp, and almost fast enough. I had photographers on a TV show I once did covet that lens. I loaned it out during the production to the participating photogs who wanted to use it.

For a prime:

Maybe a 50mm f/1.2 Nikkor manual focus. It's a 300 f/2.8 (at 2.8) but much closer working distance (intimate, no less.) Great for portraits on a DX camera (just the eyes and lips in focus) and serviceable for many a thing (with good placement) on a FX camera. It is a bokeh monster.

That said, on the manual focus side, if I were enslaved to portraiture, I might side with my 105 f/1.8 Nikkor. Fabulous lower contrast lens, very sharp, aching for the digital darkroom.