Picking a Compound, Not Just the Biggest Number
Four families cover almost everything sold here. Silicone and ceramic compounds are the safe default: electrically inert, forgiving to apply, and close enough to the leaders that the difference rarely shows in a case with decent airflow. Carbon-loaded and metal-loaded pastes sit a little ahead and are what enthusiasts reach for. Phase-change material is solid at room temperature and only softens under load, which is a different product with a different reason to exist. Gallium-based liquid metal is a specialist option with real hazards attached.
The conductivity rating in watts per metre-kelvin is measured on a thick sample under laboratory conditions, while the layer in your build is a few microns thick and squeezed between two surfaces that are never perfectly flat. That is why a highly rated compound applied badly loses to a mid-range one applied properly, and why mounting pressure, cooler class and case airflow move temperatures further than the tube does. Choose for the situation - a laptop that heat-cycles daily, a bare graphics die, an air-cooled desktop - rather than for the headline figure.
