the question you pose is what is the best low tier program
I too am going to say AS. Simply because AS as it stands now has 2 tiers and is attainable with less miles than most other low tier frequent flyer choices and upgrades arent rare or unheard of for MVP on a lot of routes. You also get 50% bonus RDMS...again unheard of on most silver/low tier status
What is the best program for you the OP may not be AS but IMO is the best "low tier" program
Does this apply on carriers other than AS? I usually am only flying the TPACs and the occasional China domestic flight and even rarer domestic US flight.