The best essays have are clear, correct, and convincing.
All essays should be clear. This means that every essay should have an introduction, main body, and conclusion. The introduction should explicitly tell the reader what your answer to the question is. The main body should include three explicitly distinct paragraphs that help answer your question. Your conclusion should outline the entire essay in three sentences. A clear, simple structure is easier for you to write and the examiner to mark. If the essay is easier to read, the examiner is happy. If the examiner is happy, they are more likely to reward you with marks.
All essays should be correct. By correct we mean factually true. The arguments you make need to be supported by factual evidence. This means that you need to have at hand lots of examples to deploy in your essay. If your essay has interesting facts, and you tell the reader why these facts suppport your argument, you will score lots of marks. But to do this, you need lots of facts.
All essays should be convincing. You must repeat throughout your essay what your argument/answer is. You must not sit on the fence but choose one side or the other. To score extra marks, nuance your argument.