The best final sentence is memorable and communicates a sense of closure. For symmetry, many like to restate the the main idea, but be careful not to use redundant phrases like, "In conclusion,". The fact that it's the last sentence is obvious to the reader.
Some tantalize with a hint of larger implications. Some use humor to be more memorable. You must consider the topic and tone of your essay to determine what matches best.
In conclusion, I'd just like to say that some people have accused me of not being seriously-minded. To that I would just say "clurgle blurgle snurgle gleep gleep ftang ftang olay biscuit barrel."
The very best concluding sentence is sure to be, "In conclusion, I would like to reiterate what I've already expounded ad nauseam until the bile rises in every reader's throats from boredom what I've already said in convoluted ways several times already..."
That's sure to get the reader's attention!
Ha ! " The end . " works well too.
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