Dunno if it's the whitest, but they just don't come much better than Mesmerizer.

Skating Party is pretty close to what you want. Immortality and Lacy Snowflake. Snow Flurry and Touch of Heaven are a few others. Helen McGregor is an oldie but it is White as white.

Some of the whites have too much lavendar in them for my garden, and they tend to look gray. I have that problem with Immortality. The ones that tend more to yellow look whiter in my light. A lot of people like the one Aquawise mentioned- Skating Party. I love Mesmerizer- it looks very showy in the garden.

I love Winterfest with its huge blooms and robust attitude towards my wet northwest climate.

Skating Party is my favorite.
kay

I have Immortality and it's one of the best whites I've seen!

Kenny

I have Zurich, and it is a very nice white, with yellow beard. It is supposed to rebloom but I doubt it ever will for me in zone 5.

I agree, Mesmerizer is a beauty.

Admiral's Cup - single bloom. Planted summer of 2011, moved to this bed 6 months ago or so.

Group shot. No lavender in this one, looks somewhat green until it fully opens.

Aquawise, Helen McGregor is not white, it is blue.

"Skating Party" would be my favorite. But I've not seen them all.