When the best grinding wheel for concrete, the best type of grinding wheel to use is a diamond cup wheel. Diamond cup wheels are made with industrial diamonds bonded to the edges of a metal or resin wheel, which makes them incredibly durable and effective at grinding concrete.

Here are a few things to keep in mind when selecting a diamond cup wheel for concrete grinding:

1. Size: Make sure to choose a cup wheel that is appropriate for the size of your grinder. Most cup wheels are designed to fit 4-inch, 5-inch, or 7-inch grinders.
2. Diamond grit: Choose a diamond cup wheel with the appropriate grit for your job. The higher the grit number, the finer the diamond particles and the smoother the surface finish will be. For grinding down rough concrete, a coarse grit (such as 30 or 40) is ideal, while for smoothing out the surface, a finer grit (such as 80 or 120) is better.
3. Bond: The bond of the diamond cup wheel refers to how tightly the diamonds are held together in the wheel. A soft bond is better for grinding hard concrete, while a hard bond is better for grinding softer concrete.
4. Wet or dry: Decide if you will be grinding wet or dry. If you are grinding wet, choose a cup wheel designed for wet use to prevent damage to the wheel.

Overall, the best diamond cup wheel for concrete grinding will depend on the specific job you are doing. It’s important to consider the size, diamond grit, bond, and wet or dry use to find the best cup wheel for your needs.