I think it depends on your flying patterns and what type of tickets you buy. If you buy full fare, almost any airline will provide you nice benefits.
NW/CO are great for free upgrades from all fares.
AA is, in my opinion, the way to go if you fly a lot internationally as they offer better upgrading opportunities on cheap international tickets.
If on the other hand, you fly in paid business class internationally, then you basically decide on what airlines suit your fancy. Do you enjoy BA/CX/QF, then One World is for you. Do you enjoy LH/SQ/NZ, then Star Alliance is the way to go.
All in all, not a simple answer.