Considered Alaska's program? Partners also with AA, BA, CX, and QF, as well as the six most relevant carriers you'd get from CO/NW/DL's programs (ie CO/NW/DL/AS/AF/KL). If you're not going to make an elite status and aren't going to fly niche Skyteam carriers (like SU or OK) or particular partners of CO/NW/DL (eg CO partners with BR, DL with SQ, and NW with MH), then AS is by far the best catch-all program.
Do read the small print at
http://www.alaskaair.com/as/mileagep...rs_Airline.asp, though.