You'll probably accrue the most miles on DL, as they now award SkyPesos for fare classes that don't earn full mileage on KL, AF, and AZ. However, spending SkyPesos is a challenge (if you don't have 100K to part with to do a LON-AMS r/t), and the international origination surcharge is a killer on award tickets. I've stuck with DL since moving to the UK because I will be back in the US in a year and a half. However, I probably wouldn't pick it as a long-term EU-based ST flyer.