First, this will depend on your audience and the subject of your essay. Second, you definitely want strong evidence to support your thesis. And in terms of communicating your point, you should consider your audience as you approach the writing style. In other words, if the audience is comprised of academics, a more formal style might be better received. If the audience is younger or perhaps more interested in pop culture, a less formal style might be better received. However, use your own voice primarily. Anything too formal or too informal won't sound genuine. Give the argument as if you were explaining it to a close friend (as if in your own words) and then clean it up and revise it.
These are used to persuade the audience. Such devices can be used to evoke emotion or some connection from the audience. (Thus, knowing your audience is very helpful.) For example, if you're trying to persuade children, ages 3-5, to eat their vegetables, try making cultural references to Dora the Explorer or something they can relate to; likewise, for other age groups. Besides appealing to your audience's demographic, speakers often use irony and metaphor to explain their points.
Metaphors and analogies are extremely useful when used correctly. Let's say you're giving a talk on preventative health care. You might say that if more funding goes to preventative health care, there will be less illnesses and injuries overall, thus reducing the cost of health care while increasing the general health of the entire population. Investing in preventative health care is similar to making sure your car works before you drive it across the country. The problems will be caught sooner and therefore more cheaply and more easily fixed.
Metaphors and/or analogies such as the car maintenance/preventative health care example help put arguments into more commonplace contexts, and this helps communicate the argument more effectively.
I understand it very much depends on my audience, I will be speaking to an 11th and 12th grade class. Knowing this however I am finding it very difficult finding any such a rhetorical device to fit my chosen topic of i feel that the 10 point grading scale system is much better than the 7 point grading scale system. My topic selection leaves not much room to my imagination to fill that requirement. I'm really stumped here but your insight has given me some food for thought. Thank you for your input :)