Of the 4 whites that I grow, Forsyth Summer Snow is my whitest daylily. The other 3 whites are Arctic Lace, Joan Senior, and Admiral's Braid. I had seen Gentle Shepherd at a garden center and it was very white too. Here is Forsyth Summer Snow pic. It actually looks whiter in person.

These are my sort of white ones. I don't think all of these are considered white, but Winter Treasure is the whitest.
Artic Lace
Victorian Lace
Winter Treasure
Wonder of It All

I would agree with Winter Treasure.
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Victorian Lace is a disaster here.

I have Gentle Shepherd & it is probably the whitest rounded daylily in my garden. Heavenly Angel Ice is the whitest spider type.

Winter Treasure is my whitest, too!

Gentle Shepherd hands down. Simple form but brilliant. I have it and many other whites as companions to a large number of pinks and reds. I think the color of any DL can be shown to it's best with other carefully chosen neighbors. I don't put one white near any other to stop any comparison. Next to a rich color most whites will read a nice white.

WINTER TREASURE looks like a winner. You guys should offer your pictures to the database. Much better than what they have!

wren-garden: Exactly my thoughts with whites but also with yellow. Sometimes a pop of yellow or white makes your eyes appreciate the other colors. :-)

Ice Carnival is very nice. A nice robust plant with nice big dramatic flowers.

Just planted Margot Reed Indeed last fall, and it went to the trouble to bloom before the snow. WOW! I can hardly wait to see what it looks like with a bit of time to set roots. My old favorite white, which looks white to me, is Joan Senior. Lime Frost is nice, white, and late.