There isnt really a "best" type of ram there are different speeds which in some cases can prove to be the best.
As for companies I my self use Kingston. I have 2 2gig sticks which equals 4 gigs and they run great, there fast and dont run hot.
Make sure you choose a quality brand and the correct type. Good luck.
The cheapest RAM is value RAM, that's intended for non-overclock use. However, This memory can still be overclocked 'a little bit'. This memory mostly run at default speed. (800Mhz for DDR2), which is quick enough. This uses chipset Elpida, Promos, Infineon, etc.
The best RAM is Extreme RAM, that's intended for achieving maximum performance through overclocking memory. This memory is mostly paired with extreme CPU and motherboard. This RAM is usually made of Micron chipset. A very good chip but also a more expensive one. This RAM can run faster than normal memory standard speed in its class (Example, 1066Mhz for DDR2). But also requires tweaking for overclocking.
Quality brands are A-Data, Corsair Dominator, albeit almost twice as expensive. Team Extreem, Patriot, OCZ, G.Skill, Kingston. They also all have lifetime warranty. Quality brands.