Into the Wardrobe

Mere Christianity seems the obvious one. If you are up for something a little more subtle (yet pure genius) you can't go wrong with Till We Have Faces. Screwtape Letters eventually, but it's better after you've read his more plain writings.

The Narnia books, in my opinion are his deepest, most resounding works. I would read them right after Mere Christianity. After you read these, delve into his more personal works; Surprised By Joy and A Grief Observed. These should give you a general overview.

I would read Screwtape Letters first. It's among my favorites of his writings. It really makes you see the spiritual battle that we don't even know we face every day.
The Problem of Pain is another good one, and I would read The Four Loves also.

Of course, Chronicles of Narnia are some of his best books and have a great deal of depth to them. They are a must-read.

Have fun reading!

I strongly support the suggestion of Till We Have Faces, especailly if you enjoy books that reveal deeper meaning every time you read them (which then has the added benefit of providing more reading with fewer books to drag across the world). It's the only book I've read four times.

I would go for Till We Have Faces or Mere Christianity very good works by Lewis.

My favorite is The Great Divorce because it deals so many of our spiritual barriers.