TL;DR: Pentaborane or LH2/LOX
From a purely specific impulse standpoint, pentaborane is perhaps one of the most potent rocket fuels, providing excellent specific impulse (about 340/365 seconds in atm/vac, see RD-270M) in the atmosphere. In addition, when combined with nitrogen tetroxide (RD-270M again), it can be used as a storable propellant as it is liquid at room temperature. For higher-energy combustion oxygen difluoride could also be used but it is very unstable and reacts with many materials. However, the reason why it isn't used as rocket fuel is because of its high toxicity and deadly exhaust products.
The lithium/fluorine/hydrogen combination does give 542 seconds of specific impulse, which is very good for a chemical propellant, but it is relatively impractical due to the difference of temperatures, with the addition of a toxic exhaust product and poor storability (lithium metal is liquid at 180 C, fluorine reacts with almost everything, and hydrogen liquifies near absolute zero, making the temperature differences near-impossible to fix). Plus, the tankage and properties of such a tripropellant may be a plumber's nightmare.
On the other hand, LH2/LOX (aka hydrolox) is a great rocket fuel for vacuum-optimized engines, providing up to 460-470 seconds of impulse in a vacuum. Many upper stages use this propellant mix, while a few launch vehicles use it as a lower stage (Ariane 5, Delta IV, Space Shuttle) but the engineering for a lower stage hydrolox engine may be more difficult due to expansion of the exhaust.
What about stuff that isn't a chemical propellant?
Again, this is from a specific impulse standpoint and not from practicality. Also this is based upon current to very near-future technology (maybe up to 50 years ahead), so antimatter-powered rockets, fusion drives, and whatnot are out of the question. If you want the best specific impulse here, use ion engines.
Ion engines (and the VASIMR) mainly use electricity (lots of it, up to several MW) and consume very little propellant for very high exhaust velocities at the cost of very low thrust, which only makes it applicable for vacuum operation. These engines can potentially reach up to 20000 seconds of specific impulse, and can operate for years without malfunctioning. However, ion engines are usually only used in small spacecraft as the tiny thrust cannot provide much acceleration for larger payloads.
The similar VASIMR may have applications for larger spacecraft or even manned spaceflight as it can trade efficiency for thrust, ranging from 1000 to 30000 seconds of impulse as thrust decreases. However, this is still in development and has not seen flight yet.
The last form of propulsion that I'll be talking about is nuclear thermal rockets, where a nuclear reactor heats up liquid hydrogen to very high temperatures and ejects it out of a nozzle, potentially giving up to 800-1000 seconds of specific impulse as well as relatively high thrust (up to 250 kN). However, such an engine is very heavy and brings the risks of reactor meltdown/failure and radioactive contamination. Although prototypes were built in the 1960s, they never saw spaceflight.
Hopefully this gives enough details about efficient rocket propellants. If you want me to give details on any other fuel, let me know.
And if you want supreme efficiency, use a kraken drive.