Most important? It's really difficult for me to distill a program down to one benefit.

I used to be an "upgrader" as RustyC mentioned, on AA. Earning miles (on most every fare -- that's important) and redeeming them for upgrades, not free travel, was the most important thing. Since AA had decided to charge a $500 roundtrip upgrade fee, plus the miles of course, I'm now a "free traveler" on AA. Instead of redeeming 50,000 miles + buying a ticket to get into international business, now I'll just redeem miles for a free ticket and AA can forego all of the revenue from that trip.

Elite benefits are also very important (esp. priority standby and free/cheap domestic upgrades).

So, yeah, that's not one most important factor. Sorry.