From a FF point of view, I definitely agree with you and would put ST 2nd and OW third (although one's particular travel patterm could affect the ranking - if the only European destination desired was LHR, OW might jump to 1st, for example).

From US' point of view, which is dollars and cents, I'm not sure which would be ranked first - I just don't have any info to base a ranking on. US doesn't offer much connectivity with any alliance non-U.S. carrier at any of it's hubs - BA at PHX/PHL, AF at PHL, LH at CLT/PHL so depends on those 1 or 2 flights/day plus it's own TA flights to get the majority of it's TA or TPAC passengers from any alliance.

From an alliance point of view (who would invite US to the party?), I don't see much difference. US fills a hole in any of the alliance's U.S./this side of the Atlantic coverage For *A it's primarily the Caribbean/LA and the Southeast. For OW it's primarily the mid-Atlantic and maybe some Southwest from PHX. For ST, it the Caribbean and to some degree New England. US' TA network is probably more important to *A (many connecting points to *A European carriers in Europe). Then OW (offers non-stop TA to more European destinations than AA, but lacks much connectivity with BA). DL has plenty of non-stop service to Europe so US' TA network wouldn't add much of anything to ST.

Jim