As an experiment I ran all the CPU stressor methods in stress-ng for 60 seconds across a range of devices, from small ARM based Raspberry Pi 2 and 3 to much larger Xeon desktop servers just to see how hot CPUs get. The thermal measurements were based on the most relevant thermal zones, for example, on x86 this is the CPU package thermal zone. In between each stress run 60 seconds of idle time was added to allow the CPU to cool.
Below are the results:
As one can see, quite a mixed set of results and it is hard to recommend any specific CPU stressor method as the "best" across a range of CPUs. It does appear that the mix of 64 bit integer and floating point cpu stress methods do seem to be generally rather good for making most CPUs run hot.
With this is mind, I think we can conclude there is no such thing as a perfect way to make a CPU run hot as it is very architecture dependant. Fortunately the stress-ng CPU stressor has a large suite of methods to exercise the CPU in different ways, so there should be a good stressor somewhere in that collection to max out your CPU. Knowing which one is the tricky part(!)