Fivenines

The question you should ask yourself is WHY do you want lower temperatures. And what are you actually aiming for?
 
The VRM (the dc-dc converter that converts 12v to whatever voltage the cpu needs) has components that are designed to work at up to 125-150 degrees Celsius, but most motherboards have temperatures sensors and heatsinks on that circuitry to keep the temperature below around 100 degrees Celsius. At more than that, over longer periods of time (like days to weeks of running at over 100 degrees) the circuit board material can be affected.
So most motherboards will keep the temperature of that circuit at around 80-90 degrees when you're using the CPU to the maximum.
There's minimal benefit to running that circuit at lower temperatures, you're not gaining much.
 
As for the processor, the Ryzen processors will work up to around 80-85 degrees before they start to throttle, to reduce their frequencies to get cooler. Again, whether it runs at 40 degrees or 60 degrees, you don't get more performance. The motherboard will adjust fan rpm to keep the noise low instead of keeping it cold.
 
Anyway, to answer question, the heat producers on motherboards are the VRMs (between cpu and IO shield and sometimes above socket, for the part that powers the integrated graphics and internal SOC part of the cpu), followed by memory vrm, chipset and then stuff like 10gbps ethernet chips. 
A cooler motherboard will have a large heatsink over that VRM area. The bigger the heatsink (and with more fins, not just a brick of aluminum), will spread heat faster and cool faster.
VRMs with more phases (more chips under the heatsink) could be cooler, simply because the heat generated is spread across more chips, and the heatsink is larger to cover all those chips... but it's not always the case ... for example you could have a motherboard with 4 phases but which uses very high quality chips that are very efficient, or you could have a motherboard with 6 or 8 phases but which uses much cheaper less efficient chips so the heat produced could be same or higher.
 

Get one with good VRMs, and that is pretty much it

While this is true, you can also be conscious of air flow. 

and well cooled ones of course
 
get one with a vrm heatsink

You can change the fans, and I don't think you can replace the pump, rather you just have to buy a whole new AIO. Most likely because I doubt those companies sell the individual parts to them

The pumps are one and done, so when they fail most folks just buy another cooler. As for the fans that come with the AIO to go on the rad, of course you can swap those out with something that performs a bit better. Try the Noctua static pressure optimized fans, I am using them in my rig!

well... good luck with it then... btw, you can post pictures of your rig in general discussion>show off your rig

However you want. I just put what I thought made sense and what was important to least important.

you don't have to... i just did it this way

How are you measuring case temps? And where? Also, check what temps you get under load. Thats the important part. Since any harm that parts would get, happens when temps are over 85C for long periods. Having low idle temps only means that your coolers are correctly mounted.
 
You can't go lower than room temp with regular cooling methods. With 20C room temp, using water cooling on both, you are about as low as you can get.

That is excellent temp. What is your ambient temps? My place ambient temps currently is 32C, it's freaking hot here lately, and with this ambient temps my idle stock clocked i7 8700K temp is 42C. Yes, I dont use Air Conditioner.
 
I downclocked my CPU at this time until the climate is getting cooler than now.

those are already exelent temps tho, buy better coolers if you want them lower. 

Airflow is EVERYTHING for good low temps.
I myself have 6 case fans.