I'm sure this has been asked previously, but with the addition of Swiss and SAA, what is the shortest path to upper tiers on *A? I'm based in YYZ and hope to get the upper tier sooner than later. AC is not exactly great so I'm hoping for a fast track on one of the others. I'm still not sure which program to join now.

Does anyone know if any of the *A carriers offer the "challenge" like AA and the others do?

Thanks!