Prime95 is generally the best stress test
But you have to do several tests
With all cores loaded, the top speed will be slower then with just 1-3 cores loaded. So you will have to run a all core test then later run the first 3 cores then last 3 cores.
This is because turbo core only kicks in with 1-3cores at load while the others enter a lower power state
It may be a good idea to just test 2 cores at a time so windows it self does not kick turbo core on and off.