Question What cooler do I need to safely put the i5 9400f on all core turbo?

An MCE -capable/enabled motherboard will boost all cores CPU to max turbo (or higher if dealing with a K-series processor) under heavy load (including gaming), yet still idle down to 800-1000 MHz (if in Balanced Power Mode) when browsing the web, watching Youtube., etc... (Mine has done so flawlessly for a little over 2 years..)
 
An MCE -capable/enabled motherboard will boost all cores CPU to max turbo (or higher if dealing with a K-series processor) under heavy load (including gaming), yet still idle down to 800-1000 MHz (if in Balanced Power Mode) when browsing the web, watching Youtube., etc... (Mine has done so flawlessly for a little over 2 years..)
Fair enough, so yea that 212 EVO Black would still do the trick although there are better/quieter coolers out there for a bit more.

If you want something that is practically silent then I'd get this.

PCPartPicker Part List
CPU Cooler: Scythe - Mugen 5 PCGH Edition 43.03 CFM CPU Cooler ($64.02 @ Amazon)
Total: $64.02
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-06-10 19:27 EDT-0400
 
The Mugen 5 Scythe has indeed been quite popular...

If one's case is wide enough to allow the 168 mm Noctua NH-D15, it's still the 'last stop' in air coolers...(although I doubt many people want to spend $90 extra cooling some of the sub-$200 cpu options....)