I plant 16ft of pole beans on 7ft tall hog panels every year, and I like a variety, both for flavor, texture, and to cover bases for seasonal growing variances. After trying many varieties, the ones that I go back to every year are: Rattlesnake, Fortex, Carminat, Red Noodle, Yard Long, and Goldmarie. If I had to limit myself to one, I would pick the Rattlesnake, at least for our Sacramento climate. It's always the first and last to harvest, heavy crops, tastes great, is more forgiving in picking size, and easier to pick due to the purple mottled color, which stands out better among the leaves.
Carminat is a nice purple bean for some variety, and is good tasting and good texture. The purple vines are nice looking, but make harvesting a purple bean bit more difficult. A bit later than some varieties.