We report comprehensive electrical characterization of electrolyte-gated field-effect transistors (FETs) incorporating organic single crystals of rubrene and several types of high capacitance ionic liquids (ILs). The specific capacitance associated with the liquid gate is exceptionally large, in the range of 1-10 μF/cm2, which facilitates the operation of devices at gate voltages below one volt. Gate-induced hole densities in rubrene single crystals are therefore on the order of 1013 cm-2, as determined from displacement current measurements (DCM) and confirmed by AC impedance measurements. Importantly, we observe a pronounced maximum in channel conductance with all ionic liquid gates which we attribute to a carrier localization effect at the semiconductor/liquid interface. Effective carrier mobility is a nonmonotonic function of gate voltage and depends on the choice of the IL. By gating with a tris(pentafluoroethyl)trifluorophosphate (FAP) containing IL, maximum carrier mobility in rubrene can be enhanced up to 3.2 cm2 V-1 s-1 at room temperature. Extensive efforts have been made to maximize the charge densities accumulated in rubrene crystals. At lower temperatures, higher gate bias can be applied before device breakdown, and up to 6 × 1013 cm-2 carriers can be accumulated at the rubrene/IL interface (0.3 holes per rubrene molecule), which doubles the amount of accumulated charge achieved at room temperature.