Best intro CD to Karlheinz Stockhausen's music?

Last night I had my (abstract) photography portfolio reviewed by a professor at the School of Visual Arts in NYC. Among other observations, he suggested listening to the work of Karlheinz Stockhausen as a departure point to understand better the photography work that lays ahead of me.

Of those familiar with his work, please suggest 2–3 solid CDs that are a good introduction to his most popular work. I’ll leave the more essoteric/complex ones for later (baby steps, you know?), but feel free to suggest those too, just clarify which is which.