What iris are most suitable for naturalizing?
All iris multiply which is usually meant by the term naturalize. Most are spring bloomers.
What height do you want? What is your setting size-wise? Home garden?
If you are looking for an iris that will form large colonies in a more or less natural setting, I'd recommend Iris cristata. But this is very low growing - almost a groundcover. For a taller form, Iris pseudacorus naturalizes well - almost too well, as it is considered an invasive species in various areas. And it doesn't need an especially moist area to spread rapidly. Best to check on its appropriateness for your area before planting.
Japanese wood iris, Iris japonica, also forms large colonies and is suited to a semi-woodland situation. Doubt it would be hardy enough for you, though.
As iris gal said, all iris tend to multiply over time but most stay within a limited area. This to me is quite different from naturalizing, although multiple plantings scattered throughout the area will provide a very similar effect.