All of them are interesting.
Heidegger is probably the strangest and most far-out. It's like sit and stare out the window in paralyzing contemplation type reading. The craziest thing about Heidegger is that if you really take the time to think about his arguments, you stand a strong chance of changing not only the way you think about the world but also the actions you take.
Hume and Kant are more direct, trying to solve specific problems. Hume pushes things about as far as common sense will take him. Kant tries to go deeper. The outcome is highly questionable, but he illustrates incredibly vexing philosophical problems in the process.
Husserl is very interesting. His views radically changed over the course of his career. His last work, The Crisis of European Sciences and Transcendental Phenomenology, is really dope and straight forward. I'd actually recommend that above all the others because it kind of picks from each and gives a great summary.