Of the DaYan+MF8 4x4, X-Cube 4, and ShengShou 4x4, which is the best? Also, which is the worst of the three?
If you're reasonably new to 4x4, then get the Shengshou. It is half the price of the Dayan, and a quarter of the price of the x-cube at the moment. It's not critical to get the very best cube for 4x4, particularly if you're not a pro at it yet. The shengshou is still a really good cube, even for an experienced 4x4 solver.
As for my opinion, I like the Shengshou best anyway.