I would say maybe a 300 f/4 or f/2.8
@STM has a few older prime telephotos that he swears by, but I believe that they are manual focus lenses. Hopefully he's around and will chime in.
....but seriously. Have you compared any of the ART lenses to Nikon's counterpart and if so, how much of a difference is there, really?
Speaking of ART. I still have a few ART gadgets laying around from my home recording days.