Well, if limiting yourself to one powder, and one powder only I'd have to go with RL-22 as well. Temp. sensitivity and all. It's been the most consistently accurate powder I've found in my rifle. And I've tested it from 129g to 160g.

Ramshot Magnum would be another good choice. As would Ramshot Hunter and one of the 4350's. I've not used 7828 much in the 6.5x55 but it's one I'd give a try if you already have a stock of it on hand.

“There are three things all wise men fear: the sea in storm, a night with no moon, and the anger of a gentle man.”
― Patrick Rothfuss, The Wise Man's Fear