My favorite, without a doubt, is Liberty Classic (left photo) but, I know my husband's favorite blue (and possibly favorite iris in general) is Merchant Marine (right photo):
It's such an eye-catcher. I have three small clumps of it next to the garage -- I think it's going to stay there. It really shows off next to the light colored siding. It doesn't seem to mind the shady spot, and it'll make a great back-drop for whatever I plant there in the future!!
Thoughts become things -- choose the good ones. (www.tut.com)