不如别人的人: it means the people who did somthing not well compared to other people,for example,A does best work, but B does not wll, then we can say,for work B 不如A . 低下的人: it means somebody is in a lower status, for example,to slave owner,slave is a 低下的人. PS.but in china now,we almost don't use this words to other person. 自卑: it is one kind of personality, and is contrary to confidence 较差的:the level of somthing or some people is lower than the standard but compared, which is not the lowest. 下级: it usually use to work position; for example, a assitant manager 是 manager 的下级 低等的：it decribes something is lower grade in structure or quality. for example ant is a 低等动物 (low animal). as u know people is altitude animal hope the above is helpful to u

i think you should say 我觉得自己不如别人 if what you mean is you feel you are inferior to others. 低下is a rather insulting word when used to describing someone. we more often use this word when saying 地位低下 or 水平低下. 自卑means feeling and believing that you are inferior to others, its a self-feeling. So you can say我觉得自己不如别人，我很自卑。 较差simply means not good. can apply to lots of things but seldom people. 下级and低等的 both words relate to rank or hierarchy. 下级is often used as a noun meaning someone who is lower than some other one in hierarchy. and is usually used as somebody's 下级 like 我的下级or他的下级. 低等的 cannot used to describe people,otherwise its a big insulting. 差的and次的 mean not good，second-class. still cannot apply to people.

Well,first of all ,I'd like to say,it must take a lot of carefulness for you to accumulate these phrases together.不如别人的人is a phrase which can work as a noun.It means a person who is not as well as others in some aspects.You know ,we all like to compare our lovers or firends or even kids with others',so if you want to express the idea that Jim is a person not as well as others,you can say"Jim 是个不如别人的人".But I strongly suggest you didn't use this phrase,because it deny a person completely,which means it will harm one's feelings a lot. As to the phrase低下的人,I have to admit that we don't use it separating from an noun.It usually comes after an noun which means one's personality or timber.If you want to say Jim is a person with poor moral character(in Chinese,we call moral character品德),you can say Jim 是个品德低下的人. When used as an adj,自卑 equals to self-abased,so when it's used as a noun,it surely means self-abasement. You can understand 差的 as bad,and 较差的means less bad.次的 is another way of saying 差的. 下级 is normally a subordinate,it's different from other phrases you ask .It doesn't show a negative feeling ,it just describes a relationship.

不如别人的人：people who are not as good as others when compared . 【she's beautiful and smart, i'm not as good as her.她漂亮又聪明，我不如她。】 低下的人：it's about rankings of people, it means the people who have an inferior status 【In the Middle Ages,a serf had a very inferior status in life.中世纪时，农奴地位低下。】 自卑：self-contempt , a sense of inferiority 【I always felt inferior to others before. 我以前总是觉得很自卑。】 较差的：the worse one 【The food at that restaurant is inferior. 这家餐馆的食物较差。】 You are right about 下级，低等的，差的，次的。

不如别人：wife yells to her husband: 我瞎了眼了，嫁给你！你各方面都不如别人！ 低下：地位低下 slave ，仆人 自卑：我家很穷，让我很自卑。我考试成绩总是倒数第一，我很自卑。 下级：opposite to supervisor. 我有2个下级。