What frequency is the current RAM running at? Just want to buy RAM rated for a similar frequency or want some room to upgrade down the road? If you post the rest of the system specs we can help you a bit better but here:
Here is some decent "Value RAM"...
Better quality and probably tighter timings (not listed on the Value RAM but I'd say it's safe to assume...
It's too hard to say what company is the "best" per-say. Overall, there
are companies that have a general history/reputation of production higher quality RAM. These are usually companies which would be considered "first party".
First party companies are those which manufacture both the memory chip and module, as well as selling the finished product. Second party companies will generally manufacture the RAM module and buy the chips in bulk through another source, put them together and market the finished product. Third party companies purchase both the modules and chips.
Obviously the overall consensus would be that First>Second>Third but I'd say it's safe to assume there are acceptions as with anything else.