That's a loaded question. Do you just mean you want the companies with the most reliable memory, the biggest companies?
Everyone will have their opinion, but the top 2 companies are most likely: Kingston and Crucial.
You have lots of other quality players out there (Corsair, OCZ, Mushkin, Giel, Patriot, G.Skill), and some value players (like PNY, PQI, A-Data).
If I'm not mistaken, I think only Kingston actually makes their own memory, maybe Crucial does too. Everyone else usually buys memory chips and controllers from some other company (Samsung, Toshiba, chinese companies you never heard of), and packages them as their own brand. Not to say that the product isn't of good quality. The good ones do their own product testing, but in the end, Corsair and OCZ might be selling you the same product, just with different packaging.