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).

> As for who makes the best RAM, it changes from one generation to the next

This reminds me of HDDs and SSDs, though I've always found RAM to be generally reliable or obviously bad, while storage can look ok for a while before it fails.

And Samsung had a good reputation for enterprise SSD but obliterated it with pretty major firmware bugs (as in self destructing bugs, which they won’t patch without a support contract). SK Hynix seems to be a smaller player in that space.

Just to add CXMT from China has been catching up as well. Although I don't believe they will be up to leading edge DRAM given the current state of things.

HBM is also used in NPUs (network processing units) to be able to buffer and process packets at line rate (e.g., 400 Gbps) in high end switches and routers.

My sibling comment has a lot of good info, but to answer your question for ddr5, SK Hynix makes the ram capable of the highest frequencies/best timings, Samsung is second and micron is third. At least this was true when ddr5 initially came out, these things could change. Its also worth mentioning that manufactures can have different grades/product lines so there is variation within a manufacturer to. My perception is that Micron and Samsung are cheaper than SK Hynix, though idk what wholesale prices actually look like.

In addition to generation, there are also differences based on capacity. I know less about DDR5 specifics, but with DDR4 you had Micron having the best chips for 32GB dual-rank sticks, while Samsung had the best for smaller capacities.

The "best" depends on what you want. For example, with desktop memory the previous DDR4 gen Samsung's B-die was considered the top choice if you wanted the fastest speeds and highest overclocking potential. Micron's rev-e die was also desirable as it was usually dual rank (technically slower, but for certain real world applications you will get better performance) and also overclocked quite well.

Now with DDR5, Hynix's A-die is considered the best option.

Dual rank was a lot faster in DDR4 days IIRC, because it allowed a memory channel to have more requests in-flight on the memory - it could be worth like +10% performance or more.

Less impactful in DDR5 because they made it a design goal to have more "ranks" by default, though it does still have a small performance benefit.

SK Hynix is winning in DDR5 land - their memory performs significantly better in all respects compared to Micron and Samsung.

Whether that matters much is debateable - maybe they get higher yields as a result (since more chips are sufficiently performant to be useful) but JEDEC specs seem pretty generous relative to what you can achieve on consumer platforms, so I somewhat doubt much RAM is thrown out because it's too slow to meet spec.

It's pretty random, though. Back in DDR4 days Samsung produced the best memory (B-die) and it wasn't particularly close - near-DDR5 speeds with lower latencies. At the same time, some of their other dies (I guess from other fabs?) was absolutely awful.

Again, I don't know how much that translates into profit though, since the performance user market for RAM is probably a fraction of a fraction of the overall memory market.