Which pair of headphones are the best DT 770 PRO, DT 880 PRO or DT 990 PRO?

All three models are comparable in terms of mechanical and accoustic quality. The description 770,880 and 990 doesn't indicate higher or lower quality. In fact the three models differ significantly in sound because of the accustic construction (open-back/closed-back) and the difference of the frequency response.