Blue Lake for me. I love how they taste, and how the color sets after blanching for freezing. They're prolific too, and I get tired of picking them by the end - but it's sure nice to have them in the wintertime.
I make dilly beans out of them too.
I also grew Eureka (wax bean) last year, and even though it seemed to be a little less flavorful to me, I put them in again this year. They make a very nice straight bean that works nice for dilly beans as far as uniformity in the jar. And mixing the Blue Lake with the Eureka is kinda eye appealing too I think. (That Royal Burgundy sounds like something to try!)