Thermal paste consists of two basic components: a carrier substance and a heat conducting filler, usually both non-conductive and non-capacitive. Metals like aluminium or copper are fabulous heat-conductors. Unfortunately, they are also electrical conductors, potentially capacitive and thus the use of metal fillers is associated with risk.
Non-electrically conductive thermal paste offers a thermal conductivity of 1 to 4 W/mK. Much higher values are often advertised, but are complete nonsense.
Despite its low thermal conductivity, paste is the best choice if both surfaces are smooth and flat because the compound can be squeezed to a very thin bond line. More interesting is the thermal resistance of a TIM (thermal interface material), as this takes into account both the bond line thickness and the thermal conductivity.