According to a test performed by a Dutch tech website, the HTC One M9 is hotter than any other smartphone, and this in the literal sense. The tests show that the HTC One M9 overheats up to 55 degrees Celsius when passing through GFXBench tests.
http://www.notebookcheck.net/HTC-One-M9-overheats-to-55-degrees-Celsius-in-GFXBench.138167.0.html