The most efficient charity is most likely your local religious charity, since it's so grassroots. If you're not part of any religious community, then I don't know. As far as secular charities go, I give to UNICEF since I don't see it wasting money on TV, radio, and magazine ads. I'm sure there are others, but I guess if you see it on TV, especially on prime time, then you know half the money's going to advertising. So ironically enough, the less well known local religious charities, though less well known, are likely a hell of a lot more efficient than the well-known ones wasting half of the money making themselves known on TV.
As for the Salvation Army, how it compares to secular charities, I don't know, but I wouldn't be surprised to find that it's probably less efficient than many other religious charities just because of the extra energy it spends on promoting itself. Again, generally speaking the local religious charity that merely collects from its own religious community is likely the most efficient since all is done internally at the grassroots level.
