Watercolor has by far been the most challenging and fulfilling medium I've ever worked with. While it's a B and a half (*technical term*) to learn... I'm still trying to master all of the nuances, there is really no other medium that has the range that watercolor provides. You can even see what I mean in the wide variety of watercolor paintings I've done since learning the process about a year and a half ago.
Here is that collection.