Question: What's the best way to fix a crack in concrete?
Answer: That's a prime example of a loaded question, because the answer depends on the circumstances surrounding the crack. First you have to determine whether or not the crack is a structural issue. Then you need to consider whether the concrete is still moving or static. Based on the answers to those questions, you can select a repair method.
For nonstructural cracks, routing and sealing may be an appropriate choice. You might choose instead to use a crack-bridging coating. If you want to “glue” the concrete back together, some type of resin might provide a solution. For example, injecting epoxy resin under pressure can reestablish structural integrity, provided the concrete is sound. Gravity-fed resin systems also are available for use where appropriate.
If the structural issues are significant, the crack repair may require “stitching” back together. This is a bit more complicated, and messy, but it can be very effective. For anything beyond a simple rout-and-seal, though, it would be worth consulting someone experienced in repairing concrete cracks. Regardless of what method and materials you choose, be sure to read and follow the manufacturers' directions.