The iMac, PS4, etc. all use pre applied thermal paste with pads in the chipset or ram. In recent years they have gotten very good up to a point where I don't upgrade any of my electronics I buy from an OEM anymore as it just is not worth 3-5°C anymore. It used to be huge with up to 20°C difference.
Some applications you just can't use paste as it will not bridge the gap from the chip to the heatsink, that is why there are pads with 0.5, 1, or 1.5mm thickness.
So my advice for you is thermal paste for the GPU and pads for ram and voltage regulators.