Take a look at this thead on best white landscape roses:

http://forums2.gardenweb.com/forums/load/roses/msg0510051230536.html

For taller Perennial plants, try Lavender Provence or Grosso. They are almost evergreen in my zone 6b climate. I also grow 8 other varieties, but these two are the tallest . For most of these varities, I get a decent second flush (going on now) if I cut the flowers early enough in the season, but not so much for these two.

There is "white out" which is not a knockout rose but is bred by the same person Bill Radler...

Sunny Knockout starts out yellow but quickly fades to white...

Here is a link that might be useful: White Out Rose