As far as degree completion goes, I do think the "Big Three" are the most flexible and affordable. As far as I know, they are the only three schools that do not require any courses to be taken from the school directly, allowing 100% of credits to be transferred into a degree. Also, they are the most affordable degrees using Aleks, fema, CLEPs, DSSTs, GREs and Uexcels.
However, if I had to suggest an alternative I would probably go with Western Governors University. Not a state school, but a private, nonprofit, regionally accredited institution that allows competency based testing and coursework to fulfill degree requirements.
WGU
If you can get financial aid, WGU is as affordable as the "Big Three".
For affordability, I would suggest Penn Foster State College or Cal Coast as DETC alternatives and Louisiana State University or Brigham Young University for regionally accredited alternatives. Also, I would suggest the USDA Graduate School, or your local community college.