Here is the American Rhododendron Society search engine. Use the multiple parameters including hardiness and whatever color you want and then ask your local nursery or master gardener or neighbor about what they recommend too:

http://www.rhododendron.org/search_intro.asp

The results from the search engine may include rhodies that are not commercially available.

The ARS keeps a list by chapters (areas/climates) too. I don't know if its been updated recently but there are 'proven performers' there. Your climate is outside my own experience, what's growing in my own gardens wouldn't help you ;)

http://www.rhododendron.org/performers11.htm

"Best" can, of course, mean quite different things to different people. The suggestions from Lily777 and morz8 are good ones, but the reality is that once you get away from the PNW where rhododendrons remain very popular garden plants and there are specialist nurseries, the selection of available varieties is very limited unless you order by mail. My own suggestion would be to see if you can find some of the hybrids created by David Leach. These were bred in the Midwest for cold hardiness and the ability to withstand the harshest winter conditions. The Holden Arboretum currently has the Leach collection and could probably help with a source where they can be purchased. There are many different Leach hybrids with varying sizes, colors and overall appearance.