I posted a thread on this before the forums crapped out, but early-model B-17s would be invaluable for the Early and Mid War arenas, as well as AvA, Scenario and FSO play.
I'd say a B-17F for all theaters (B-17G is inappropriate for any Pacific scenario) as well as either the B-17D or E for early PTO setups. Also an earlier B-24, and earlier British heavy and medium bombers.
However there are bigger gaps in the medium and heavy bomber lineup right now--the Japanese, Germans and Russians in particular--that should be filled first.