Samsung, SK Hynix, and Micron are the only three manufacturers doing leading-edge DRAM manufacturing, but there are other companies like Nanya that do older DRAM types and specialized DRAM parts for eg. embedded and industrial applications rather than PC, servers, and phones.

As for who makes the best RAM, it changes from one generation to the next, and also depends on what you consider "best": you might be looking for chips that overclock well in a desktop, or that are least likely to suffer compatibility issues and performance loss when maxing out the capacity in a desktop, or maybe "best" might mean who has the lowest-power LPDDR for your phone.

The DRAM parts made by the big three largely all adhere to the same standards (thought not necessarily all supporting the same frequencies), with the most significant recent example being GDDR6X that was essentially a NVIDIA-Micron exclusive partnership. For the most part, it's the latest iteration of DDR (desktops and servers), LPDDR (handhelds and low-power laptops), GDDR (GPUs), and HBM (more expensive GPUs).