Much of what your topic sentence will be is dependent on what your paragraph is going to say. The topic sentence has to embrace the supporting details in your paragraph. Without knowing what these are, being able to suggest a topic sentence is a challenging element. For example, if you are going discuss how different qualities are present in an "ideal teacher," one topic sentence might be, "There are many different qualities that an ideal teacher should possess." This enables that all of the details come back to this topic sentence.
Another approach would be to present an opposite point of view. In this, you would detail the qualities that the less than desirable teachers possess and then suggest that the ideal teacher exceed this. For example, "The ideal teacher sets themselves apart from other teachers in the manner in which they embody the highest of standards." What might follow would be how other teachers fall short from this ideal, and help to explain how the ideal teacher reaches this highest notion of the educational good. In the end, your topic sentence has to cradle the supporting details of the paragraph.