i'm no expert, but my personal recommendation will be the nikon 12-24mm or the tokina 11-16, both of which are great lenses. what budget are you looking at?
Well, it really depends on how wide you want. I've got a Tokina 11-16mm, very sharp. I've heard great things for the 12-24mm too.
I do use 28mm f/2.8 AIS on my FM and my dslr. But on dslr, it actually functions much like 50mm due to the crop factor.