The purpose of the present paper is to systematically study and compare the thermal conductivity of nanofluids based on water+surfactants, ethylene glycol, ethylene glycol+surfactants, and isopropyl alcohol with single-walled (SWCNT) and multi-walled carbon nanotubes (MWCNT). The weight concentration of carbon nanotubes varied from 0.01 to 1 %. In all cases, the excess of the thermal conductivity coefficient in the nanofluid with SWCNTs is significantly higher than with MWCNTs. However, it should be borne in mind that the volume concentrations of MWCNTs in this case are several times lower than those of SWCNTs. The maximum excess of thermal conductivity coefficient was registered in the nanofluids based on isopropyl alcohol and is over 50 % at a SWCNT weight concentration of only 0.2 %. The enhancement of the thermal conductivity coefficient is greater, the lower the thermal conductivity coefficient of based fluid. The effect of the surfactants on the thermal conductivity of nanofluids is discussed. The thermal conductivity coefficients of nanofluids based on ethylene glycol+surfactants are almost twice as high as those of nanofluids based on ethylene glycol. It is shown that at a fixed weight concentration, the thermal conductivity of nanofluids increases with a decrease in the nanotube length, but their volume concentration also increases in almost proportion to the length decreasing. Finally, the nanofluids with SWCNTs provide an excess of the thermal conductivity unattainable with the use of conventional spherical nanoparticles.