The best scenario is full contact between the CPU/GPU/whatever makes the heat and the heatsink. No product that is put between those elements will make heat dis

The best scenario is full contact between the CPU/GPU/whatever makes the heat and the heatsink. No product that is put between those elements will make heat dissipation get any better, as it won't change the composition of the heatsink or of the CPU/GPU surface. Now, in real life, CPU, GPU and heatsink don't have the perfect, more-than-mirror finished surface. There are small gaps and small peaks that prevent a full contact. And those gaps are filled with air. Air is a bad heat conductor. So, to reduce this bad, bad air effect, you fill the gaps with thermal paste. Thermal paste will be better than air pockets. Thermal paste will be worst than metal/metal (CPU/GPU/heatsink) contact. So, put as little as thermal paste as needed, just to fill the gaps. But put too few paste, or don't cover everything, and you'll end up with more gaps: the paste will prevent those metal/metal peaks contacts and if not spread evenly it will leave bigger gaps. If you can't know the ideal, it's better to have a little more paste (but covering everything) than to few or badly spread (leaving bigger gaps).