But if you are looking for a great nature sound series, you might want to check out Soothing Waterfall, Soothing Rain and Soothing Ocean Waves by Deep Sleep Systems. The focus is sleep, hence the artist name, but these albums creates great ambience for creative work and active relaxation too.
 

The most important thing for a series like this is sound quality. A nature sound series with really bad quality is worthless. I find the sound on these albums to be dynamic, and the placement of the microphone is very good (the sound is around you, not in the front and not in the back). When buying these titles you get MP3s encoded at 256 kbps, which is not very high but OK.

Deep Sleep Systems has with these three albums created a very good package. It soothing, like nature itself. Try it out!

Sample the albums here: Soothing Ocean Waves for Sleep   ,  Soothing Rain Sounds for Sleep and Soothing Waterfall