I will go with Karl Jaspers, a Catholic existentialist similar to Kierkegaard, though much more readable. His Way to Wisdom was the first philosophy book I read, and it really turned me on to the discipline. I've always found Sartre inexplicably dark, Camus too, and Kierkegaard practically unreadable, but less heavy. Jaspers is like the slightly tipsy uncle who's the most fun and all the children adore. He isn't as eloquent Sartre, but he can be a lot more fun in academic works. My admittedly fuzzy memory of Way to Wisdom is that it is saying, to a certain extent, "So what if there is no inherent purpose to life, are you really going to let that stop you?" Unfortunately, Jaspers didn't leave many works behind that have been translated to English. And I can't say at this point that I totally agree with his philosophy. But his approach to it, focusing so much on the end goal (wisdom) is something that really appeals to me.