I like to use my miles for flights from WAS or BOS to Germany, and have found UA best for this purpose. As far as BOS is concerned, UA miles can be used on non-stop flights to FRA and MUC (on LH) and to ZRH on Swiss. AA and its partners do not have non-stop flights from BOS to Germany, and its main partner, BA, has prohibitive surcharges on award travel. Though UA and US are currently in the same alliance, UA offers OW award travel at half-price in miles, while US does not.

I would rank AA second, at least for my purposes. I am flying to Germany on AB (to MUC, from DRS) with AA miles next summer, but have found it difficult to book award travel from JFK to WAS for the final segment of my return flight.