A good topic sentence in an argumentative essay will be a thesis statement (topic + argument). While topic statements and thesis statements can be presented in separate sentences, one concise way to approach an essay introduction is to put the two together.
If you put them together your thesis statement will preface both the subject of the essay and the argument the paper will take up regarding that topic.
"The thesis statement is where you will let your readers know what position you will take on your topic. When you write your thesis, don’t be shy: make a bold and factual statement that expresses your position" (eNotes).
Most academic writing takes the form of an argument. Writers set out to prove a point of one kind or another - interpretive, opinion-based or research-oriented. The goal of the writer is to demonstrate that his/her point of view is supported by evidence (which can be textual evidence, anecdotal evidence, scientific evidence, statistical evidence, logical proof, etc.).
Here is an example of an interpretive argument thesis statement:
The character of Lennie in Of Mice and Men presents a central irony in the novel as he is intellectually "innocent" of malice yet also guilty of multiple acts of violence.
This thesis presents the topic (the character, Lennie) and outlines the argument that the paper will make regarding this topic.
If you are writing a traditional persuasive essay, the thesis statement will be similar. Here is one example:
College tuition increases are far outpacing inflation in other areas, resulting in a debt crisis that needs to be resolved before it implodes the financial market like the housing bubble did in 2007.
Again, this thesis presents a topic and an argument relating to that topic, effectively prefacing the contents of the paper.
Thesis statements, when they work, will always orient the reader by communicating a topic and an argument and also offering a brief overview of the shape that argument will take.