Too many people here are captivated by the bleedin' obvious. We realise if you slide into your 747-436b that the best altitude is the one on the piece of paper marked best altitude.
The interest in this discussion is why isn't the RB211 bigger/smaller, designed with more/less stages to spin faster/slower with a higher/lower by-pass ratio etc etc etc.
My take is still this:
1 pressure differential on the hull
2 exhaust gas temperature
3 mach effects
And ... fuel efficency is not the same as efficiency which is normally outputs divided by inputs. Fuel only accounts for about half the (financial) inputs for a jet which is why you don't normally fly at the most economic cruise speed.