Your first and some would argue most important client is your boss. Now, before you go on about how you are really working for the entire organization and want to help everyone (which is absolutely the right way of thinking and acting) take a minute and think about the opening statement a while longer.
Your future success is (in part at least) directly correlated to your boss’ satisfaction with your performance. Having said that, it is important to keep your exchanges and your actions in mind. Even if you disagree with the direction he or she has taken, publicly, you always need to support them and act in a way that makes them look good. Your career is a marathon, not a sprint and you need to plan and manage it strategically.