The Coolermaster H500M case was designed for water cooling, and contains some dedicated mounting points for pumps and other gear. However, the documentation and marketing materials are not very clear about supported radiator size. So, here's what fits:

• Top-mounted radiator: Up to 240 mm. You can't go any longer than this as the radiator will clip into the top motherboard components.
• Front-mounted radiator: Up to 360 mm will fit if you remove a panel on top of the hard drive bay / power supply shroud, to allow the bottom of the radiator to pass through into the lower area of the case. However, a 280 mm will fit without removing the panel.

I used an Arctic Liquid Freezer II 280 mm. The radiator on this AIO is a lot thicker than most other brands, and it fits comfortably in front-mounted configuration. As the airflow through the lower shrouded area of the case is restricted, I don't recommend 360 mm units with this case, unless you are willing to remove the shroud (which is not a bad idea). A review by Gamers Nexus found roughly equivalent performance between the Arctic 280 / 360 models anyway, so save your money and time and just use a 280 mm.

The performance of the Arctic Liquid Freezer II 280 mm is really good, I am using it to cool a 16-core AMD Ryzen 5950X. In an all-core Prime95 torture test the average temperature is maintained around 55 C.