The basis of ethics is what is good for the life of the individual, i.e. what nourishes or promotes life activities thus actualizing that being's potentials. Another way of putting this is that what is right for the individual is the fulfillment of that individual's natural function as the sort of living being it is.
When it comes to human beings, that natural function is most significantly the effective use of one's own psychology for the purpose of attending to the issues of life. Effective here means, at root, the ability to understand and to act on one's understanding. And so, at root, human ethics is properly about acting rationally and in ways that promote rationality.
Ever read Nicomachean Ethics?

This sounds to me like Aristotle's function argument. It procedes thusly:
1) The virtue of a being deals with its function (purpose derived from nature or from the gods).
2) The function of a human being is to use reason well.
3) The virtuous human is the one who cultivates and uses his reason well.
Here is a challenge to the function argument, and correct me if this is
not the position you are asserting:
I am not convinced that either 1 or 2 are true. What is good for something is for that thing to fulfill it's function? A car's function is to drive, but driving is certainly not beneficial for the car. It devalues it and eventually leads it to break down. Also, how do we arive at the idea that man's function is to reason well?
Let me stress here that I don't mean to imply that ethics has nothing to do with emotions, desires, passions, etc. Human psychology is complex, and so we must always keep the full context of our psychological needs in mind, especially as they pertain to rational action. For instance, we need to be sane in order to face reality, we need love in various forms in order to find the motivation to act, we need self-respect in order to feel confident in our rational activities, etc.
Skipping a few pages (this is a large subject), I'll mention that I think that virtue ethics has the best approach out of the schools of ethics because it stresses the agent over the act, which I think cuts closest to how ethics is used in real life. And so rationality would be the basic virtue, with other virtues either specific aspects of reason or needed habits that support qualities of our psychology that are a means to rationality.
It is true that real ethics must always stem from the virtue of the person. Whether or not a person has a virtuous
character determines his overall virtue (over his actions, etc). I'm not sure that rationality, or prudence as Aristotle called it, is the chief virtue though. If you think that Jesus had anything valuable to say then I think that this is worth consideration: When a man asked him what the chief virtue was (What is the most important commandment?), Jesus replied: "Love the Lord your God with all your heart, mind, soul, and strength, and the second is like it -- love your neighbor as yourself. All the law and the prophets hang on these two" (paraphrase). Perhaps the chief virtue is love. That makes sense to me.