Skirt steak is the best cut of beef for steak fajitas due to its marbling, thickness, and surface area. Its intramuscular fat delivers a rich, beefy flavor while maintaining moisture during grilling. The ample surface area of skirt steak allows for absorbing marinades, elevating the fajita experience. The relative thinness of skirt steak also allows for quick and even cooking, producing a perfect sear on the outside and a medium rare inside. Flank steak is a great alternative for those seeking a less expensive option.