The best RAM for your i9 9900k motherboard is the RAM that is designed for the motherboard. Not all RAM is compatible with all motherboards. You need to find RAM that is specifically designed for the motherboard that you are using. There are many different types of RAM on the market. You need to find the type of RAM that is compatible with your motherboard. Some motherboards will require DDR4 RAM while others will require DDR3 RAM. Make sure you find the type of RAM that is compatible with your motherboard.