In a concluding paragraph of an essay, what would you put in the first sentence?
In general, what you want to do with your conclusion is to wrap up your essay and reinforce what you have said. Your first sentence should get you started towards doing that.
One thing that you could do with your first sentence is to refer back to your introduction. Your introduction and conclusion should have many of the same ideas. If your introduction argued that Lincoln was an overrated president, you might want to start your conclusion in the same way by saying something like "Although Lincoln is almost universally revered, he was not the greatest president the country ever had.
Another thing that you could do with your conclusion would be to link your argument to some related and interesting topic. So, if you have been arguing that Lincoln was overrated, you could say that we might want to think about why we overrate some presidents and underrate others. You might say "Given that Lincoln is overrated, we may need to think about the ways in which we determine which presidents are great." By doing this, you can give your readers the sense that they have finished your argument and have something to think about going forward.
What you do not want to do with your first sentence is to say "In conclusion..." or "In this paper, I have..." It is better to be a bit more subtle than that.