Actually depends on the route, and the phase of the moon too I guess. I ended up flying on AA for a few of my routine DEN-BOS laps this summer. During that time UA and US were asking about $420-450 R/T, NW and DL were at about $350 or so, CO was about the same with the problem of 50% EQM for non-OnePass ST programs (NW in my case). AA was $198-218.

I ended up going with LAN rather than AA as the program to accumulate into given I was flying LAN Argentina later in the year on a fare that doesn't earn on AA. But really, YMMV in terms of where to accumulate.

Your choice of where to accumulate might end up based on some oddball thing unique to your location, your planned paid travel (like my AEP-IGR LAN/4M trip) or planned awards. No one-size fits all.

AS does sound like a good program overall. Turns out they're a partner with LAN as well as NW, CO, etc. So I may just end up using my Chilean kilometers for a trip to Alaska. Or start flying AS and accumulate more into LA from their flights. All these partnerships make for weird bedfellows, don't they?

If the AS program meets your needs, with the restrictions on award stopovers etc. that others have pointed out, it could well be the best bet given the number of different RDM/EQM partners it has in different alliances.