Skywest has benefits on United, Delta, American and Alaska. American and Alaska are base specific. All CRJ pilots get United and Delta. E175 pilots only get United but Delta flying is coming this summer.
I think Skywest is the only carrier where you can potentially have benefits on 3 majors at the same time.
Disclaimer: American and United benefits kind of suck for your family members due to segment fees and inability to list for first class on American. Delta is the best benefit.