If I need to attach a heat sink to a planar MOSFET, to which terminal should I attach it to?
It really depends on what all the terminals are connected to.

You'd probably want to connect it to the body because it has the most surface area (or in this case drain). The body is usually the best thermal pathway as there are only small leads attached to the source and gate.

Here is a pic of a mosfet that I dencapsulated  (all it takes is a bit of warm nitric acid be careful), they connected it to the drain with thermally conductive epoxy. Most of the heat will be dissipated in the small square and out  through the copper base/drain.