I love white flowers the most! And certain varieties of flowers just don't seem to have them in white. But, if you look really carefully (or wait long enough for a hybrid), you may eventually be successful! One example of this is a white day lily: Gentle Shepard
And another one is the Snowdrift White Marigold!: white marigold
I grew this from seed this year. It took over three months to get my first bloom! Now there are several dozen ready to pop! The timing is actually great because many annual summer flowers are spent and its a little too early for the mums.