I take it you're looking for something other than flank? Flank steak, when marinated, grilled, and thinly sliced against the grain for fajitas is quite tender. Another trick to getting a tender piece of meat is to use a Jaccard tool which by testimony of several members of the board can make even the cheapest, toughest, sinuey meats cook up tender. If you prefer to substitute a more expensive meat for tenderness, you can marinate, grill and slice almost any cut of beef you like for fajitas. Chicken and pork also make fantastic fajitas.
I have trouble consistantly getting Skirt Steak from my butcher, but he recommended Flap Steak which is what he uses for Fajitas and I have been hooked ever sense. Another advantage with Flap Steak is that it is much less expensive than skirt steak and just as tasty - I just fixed some fajitas last night!