Obviously people aren't equal in the sense of identical. They are different in height, in penis size (if they're male), in strength, in various sorts of intelligence, in appearance, in personality. You can even look at particular groups and find differences in average scores for various measures, though usually individual variations are larger than differences between group averages. And it's individuals who matter.
But with all the things we can measure about a human being, how can we make one number of them, and say that one human is superior to another? Superior in what? How do you rank someone who is good-looking and healthy against someone who is neither, but is particularly intelligent? There are thousands, tens, hundreds of thousands of measures.
When the concept of equality was first urged, it wasn't any of those measures I've mentioned that was uppermost in the philosophers' minds - it was birth. If you were born into the nobility, you would be treated very differently by the state than the common herd. Now some of the nobility had originally earned their titles through noble deeds - but it was apparent that nobleness of character was not always passed on through the eldest son. The upper class was a separate group, and they carefully avoided marriage with commoners, but the philosophers saw that, despite their different manners and different dress and different names, the content of their character was no better than that of the common folk. Hence, it was argued, all people should be considered as on the same level.
How can we possibly go beyond this? How can we consider the people of one nation superior to another? What could we base it on? Let us not forget also the effect of culture. Culture is not the same as race, but cultures also differ, and those differences easily give rise to perceptions of different character. When did a lot of travelling (over 20 years ago), it was very evident that in Italy you had to count your change carefully, and most of the time shop-keepers would try to cheat you. This didn't happen in other nations I visited. Did this mean Italians were less honest? In a sense yes, but it was because the culture censured that sort of dishonesty less severely. In some parts of Africa, westerners are seen as dirty, because they blow their nose on a handkerchief and put the dirty thing back in their pockets and carry it around; and rude and uncaring, because they don't say "sorry" to strangers they happen to see in trouble they haven't caused. We don't think we're being dirty or uncaring, because we're following our own culture.
We are conditioned by our cultures, by what other people around us do and expect us to do, more than we like to think. When the rules break down, when we can do nasty things and get away with it, it's surprising how thin the veneer of civilisation is seen to be. Not everyone, but very many people are quite prepared to do bad things. There's a famous psychological experiment in which many people were prepared to give someone else what they thought was a lethal electric shock if they were asked to do it. And in real-life situations, when law and order break down, people of any nation will do bad things, as we've seen in Abu Ghraib.
I don't think any of us can consider him- or herself superior to other people. We don't know what we would be like with a different set of people around us, different upbringing, different culture. Nor is there really any reason to compare ourselves with others - that's not our job, and we don't have the knowledge to do it. We can certainly make judgements about aspects of culture - we can say, for example, that the attitude to honesty in Italy isn't as good, or that it's a good thing to indicate you care to all people you meet in distress. Cultures are not fixed and they change. And we can usefully make judgements about ourselves and try to make ourselves act better, and that's quite enough for a lifetime.
Hmm, I'm rambling.