Originally Posted by
pinniped
YX
WN
Although NW doesn't have one-way awards, they are unique in that they'll allow you to pair one direction that has a standard award seat with one that does not, resulting in a 37,500 mile award (in the case of a domestic U.S. coach ticket).
NW, DL and CO all do this if I'm not mistaken. I wouldn't add any of these three carriers to the one-way award list since the cheapest award ticket will generally be 100% of the standard roundtrip coach award ticket whether for a one-way or roundtrip itinerary.