Either pressure washer or simple putty knife to scrape out the majority of the crud and then angle grinder with wire cup brush. On rehab project grills, if the burners are in decent shape, I also suggest that you hook it up and run it on high for about thirty minutes or so to burn off all the grease and liquid goo that the previous owner left in there. I even put a sheet of tin foil over the cooking grates so the cook box gets extremely hot and burns everything to dust. If the grill is really bad, it will likely start a flare up, but that just helps to burn up all that grease in the bottom.
Then, hit it with the wire cup brush and angle grinder. If you are just trying to clean up a grill, the pressure washer is probably fine, but for resale, they love to see that shiny clean cook box.