68040 @ 40 MHz runs the hottest, hands down.
It uses a 5V core, has a high transistor count and unlike its lower voltage successor does not use a static design either. Later revisions ran a bit cooler and there were even 3.3v parts but I think these were based on the EC version.
A really good source for the information you need is the CPU user manual for each component (downloadable as pdf from motorola).
They include the thermal characteristics of these chips at different clock speeds and in different packages.
