It's going to come to preference at a certain point. All three will be fine but they are somewhat different styles of rifle. I'm not an AR guy and would probably go with the Mini-14. Ruger is selling the 30rd magazines again. The Saiga will be good too. However, you'll most likely want to convert it to the AK set-up sooner or later. (Also, the magazines aren't always the easiest to find.
All that said, if I was going to make a "general recommendation," I would say to buy a S&W M&P-15, replace the standard grip and buy some P-mags.
All that said, if I was going to make a "general recommendation," I would say to buy a S&W M&P-15, replace the standard grip and buy some P-mags.