This is not an easy question to answer for me Koen! I test, and have tested very many whites. Not just because they are commercially the most interesting for cut flowers, but probably mainly because my personal preference lies with the purity of white flowers.
I can name a few here, but there certainly are a lot of ex aequo rankings for me and I should also note that I have a lot of whites that I think real highly of but have not grown to maturity just yet.

To name a few:
– Madame Claude Tain is the best commercial cut flower for me, always has been and very likely always will be. Very high production, excellent bud presentation, excellent vase life and when I put dozens of different whites in my house it always stands out – whiter than white.

– Dr. F.G. Brethour is another excellent white cut flower for me.

For the future there are a handful of nominees for me, of which at this point I shall only name Luxor – an excellent bomb of extra heavy quality.

– Greenland is another favourite, but on a whole different level because it’s not a lactiflora but a herbaceous hybrid peony. The very dark green foliage sets off so nicely against the handsomely shaped double white blooms with its very appealing perfectly rounded petals. (picture of an immature bloom from a plant in its second spring).

– For use in the garden, a current favourite is Steve’s Choice. Dwarf-ish, very compact with a very good stand and a prolific bloomer.