If the purpose of the rebuild is to bring the engine back (or close as feasbily possible) to it's original performance, what are some things that should always be replaced?

This obviously depends on the condition of the engine, so let's say a 300,000 km engine which had regular oil changes it's whole life.

Gaskets, seals, belts, bolts, etc. yes you should replace, but how about components such as valves, valve springs / hydraulics, piston heads, connecting rods, camshafts, piston rings to name a few?