Yeah, I suppose you could consider a purely hypothetical flight, but as others have said, that depends on a whole range of factors, like plane type, fuel, but generally speaking, the longer the flight, the more efficient.

Although in reality and for almost all practical cases, efficiency is not an end in itself for airlines, but just a factor to consider, as in how much that can contribute to the bottom line. You have to factor in the passenger load factor, price differentiation and whole host of other variables. A airline that is really efficient for a given distance but is nearly empty won't do much good.