We demonstrate the highest efficiency (∼80%) second harmonic generation of joule level, 27 fs, high-contrast pulses in a type-I lithium triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate gives a maximum efficiency of 26%. LBO thus offers high-intensity (>1018-19W/cm2), ultra-high contrast femtosecond pulses, which have great potential for high energy density science and applications, particularly with nanostructured targets.