Linda,
You are pretty safe with whatever Value Resort you select. But if I had to choose a favorite, it would be
Pop Century. It is more centrally located to the overall Walt Disney World property than the three All-Star resorts, often shortening travel times a bit. Also, the grounds are adjacent to a small lake. It gives you some alternative views, and a nice place to go for a walk or for some quiet time.
Finally, as Pop Century is roughly 2/3 the size of the 3 All-Star resorts combined, you have slightly smaller crowds to contend with when it comes to getting bus transportation. This is less of an advantage than you might think, but it is still a bit nicer.
I hope that this helps? If you have any other questions I can help with, please feel free to ask me directly from my bio page!
Have a magical vacation!