I get asked about the best kind of butter you can get at the grocery store frequently.
Unless you have access to someone making local butter from grass-fed cows then Kerrygold USA is probably the best option.
Most stores are carrying it now and the price has been dropping over the past year.
Grains are inflammatory for cows, if they eat them then the inflammatory fats will also be found in the butter or milk produced by that cow. Grass fed is a much better and more natural choice.