I took apart two of my Genesis controllers to wash them when I found out why one controls much better than the other. The dpads are completely different.

Left="good" pad, Right="bad" pad







The controller on the left has the type of dpad that's on the 6-button controllers and Sega Saturn controllers (where the dpad and the white part "sandwiches" the casing). It controls much better than the one on the right, especially when hitting diagonals.

The "good" controller also has the case/buttons in matte black and has a textured feel, compared to the "bad" controller with it's glossy case/buttons and smooth feel.