I found this question really hilarious, and I don't mean that in a derogatory way. I studied philosophy as an undergrad, and then as a graduate student at Stony Brook University, and to be perfectly honest this thread is full of a surprisingly good overview of a variety of ethical systems. I won't comment, I just want to say I appreciate you all.
If you are genuinely interested in this question, it is the central problem and point of discussion of, probably, the single most influential piece of philosophy ever written - Plato's Republic.