What is the most important purchase for a recording studio?

We've been saying this for years, and now the pros are going on record... Take it directly from Mastering Engineer, Ian Shepherd, after learning your craft the best investment you can make in your room is monitoring and acoustic room treatment. (The entire video is worth watching, but skip to 22:10 if you are in a hurry)

Now the only question left is "How many acoustic sound panels do I need?"