What temperature measurement tool should I trust?

Thigoron,

The i7 4790K inherently runs hot for four reasons:

(1) Unlike other Generations, 4th Generation processors have a Fully Integrated Voltage Regulator (FIVR) on the substrate, which increases package temperature.

(2) 3rd through 6th Generation processors do not have a soldered Integrated Heat Spreader (IHS); they instead use a Dow Corning Thermal Interface Material (TIM), which is less thermally efficient.

(3) The 4790K is a factory overclocked 4770K running 500 MHz faster at higher stock core voltage, which increases core temperatures.

(4) The 88 Watt 4790K uses the same stock cooler as the 84 Watt 4770K and the 77 Watt 3770K.

The stock cooler is the majority of the problem. Although your ambient temperature (23C) is close to normal (22C), you have a CrossFire configuration with axially cooled R9 290's exhausting and recirculating heat into your case. Even at idle, this combination will cause all the temperatures inside your computer case to run higher.

Here's the recommended operating range for Core temperature:

80C Hot (100% Load)
75C Warm
70C Warm (Heavy Load)
60C Norm
50C Norm (Medium Load)
40C Norm
30C Cool (Idle)
25C Cool

Your screenshot of Intel Extreme Tuning Utility shows CPU Utilization is not at dead idle, which should be less than 3% CPU Utilization. It also shows that your core voltage and core frequency are not at idle.

Does your CPU idle down to less than 3% utilization under the Performance Tab in Windows Task Manager ... and 800 MHz at about 0.800 Volts in CPU-Z? If not, you won't get the lowest possible idle temperatures for your hardware configuration.

Although Windows has dozens of Processes and Services running in the background, excessive CPU utilization at idle is typically the result of "tray-trash", which are unnecessary startup add-on's from software installers. Windows 10 Live Tiles can also contribute to CPU Utilization.

Try CPU-Z - http://www.cpuid.com/softwares/cpu-z.html

Without knowing the details such as your CPU Fan Curve in BIOS, which case or how many fans you have, or how carefully you've managed the cables, I would suggest that you upgrade from the stock cooler to a more capable cooler.

If your budget is tight, the highly popular Cooler Master Hyper 212 EVO is an inexpensive go-to, however, here's some other choices: Alternatives to the Hyper 212+/Evo for budget cooling - http://www.tomshardware.com/forum/id-2705157/alternatives-hyper-212-evo-budget-cooling.html

You can also try manually lowering core voltage settings in BIOS, which will in turn lower core temperatures.

Stickies are located at the top of each Forum; you might want to read this one: Intel Temperature Guide - http://www.tomshardware.com/forum/id-1800828/intel-temperature-guide.html

Also, on behalf of Tom's ... welcome aboard!

CT