Just pick anything with a long track record of success on 50 BMGs. Swarovski made a 10x42 specifically for that purpose. The SWFA fixed power and Nightforce NXS have done very well. So has the Leupold MkIV.
My Burris lasted 9 rounds on a 50 before the + became a X.
I think I have a swfa variable on it for right now...