Let's start with some general rules for finding good quotations: look at the most exciting parts, the most important parts, and the structurally significant parts. In most books, this would be the beginning and end, either of the whole book or of each chapter. In this book, you would also look at the sections in the letters, which seem to give more direct access to his mind. Finally, actual spoken dialogue yields fun quotations.
If I had to choose some, I'd start with one from the first chapter: "If you make it out alive, give me a call, and I'll tell you how to get the boots back to me." This is Gallien talking to Alex, and shows how casual folks are about life/death situations here.
I also like McCandless' farewell from Chapter 18: "I HAVE HAD A HAPPY LIFE AN D THANK THE LORD, GOODBYE AND MAY GOD BLESS ALL!"