Hoya now sells, ND 16, ND 32 and ND 64 in their Pro line. The 32 and 64 are new and they have been selling the ND 16 for about
a year now. I have the Hoya Pro line ND 16 and it's a very good filter. No color cast and slim. They are marketed up to 82mm.
If you need larger and want a screw in, you need to look to B+W (Schneider) or Heliopan as they make filters at least to 95mm. I
recently tired a Format 105mm but it had the color cast issue that was already mentioned in this cast strong Magenta.
The only issue with the Lee Big Stopper for me, is it's not large enough for some of my glass @ 4 x 4. The latest versions of it seem
to be very clean without colorcast. Hightech is now making a series of filters to fit in the Lee holders. They make a 150mm x 150mm
which will fit in the Lee holder for the Nikon 14mm. In that 150mm x 150mm series, they have a very good line up of ND, from 0.3 all
the way to 1.2 and then a 10 stop like the Lee. I am waiting on a Hightech 150 x 150 ND 1.2 to see if it has any color cast issues.
As for the graduated question, I have not ever used a grad ND, the only ones I have looked at were too thick and would have vignetted
with shifting or any wide angle lens. Singh Ray seems to get great reviews with their line up however.
Paul