Most fuel-efficient twin?
Probably, though my compatriots won't thank me for saying this, the 787.
I only mention this because the OP hasn't specified his mission in sufficient detail to answer more clearly.
What load?
How far?
Is speed important?
Field performance?
OEI ceiling?
Overwater speed?
...and so on.
(the 787 is quite nice to fly too, as it should be, though I'm sorry they've handled pitch-power couple the way they have and I'm not sure that Boeing have done all they should with some systems; the performance calculation looks like something from the 1980s).