The two most common and scholarly signal phrases are "Smith suggests that. . ." and "Smith argues that. . ."
The examples above are both fine signal phrases, but a research paper that contains many in-text citations can become rather tedious to read if every quotation is introduced in the same manner, for all writers tend to develop their own particular writing style habits.
In other words, writers find words and phrases that become favorites, and develop a tendency to use them frequently. The signal phrase often proves to be an instance where this repetitiveness occurs.
After you have a rough draft of your paper check all your signal phrases. If you discover that your favorite signal phrase verb seems to be “suggests,” then edit your paper by varying the verb. Although this is a simple revision tactic it is very effective, and will help to turn a somewhat tedious prose style into an interesting one.
Acknowledges, adds, admits, agrees, argues, asserts, believes, claims, comments, compares, confirms, contends, declares, denies, disputes, emphasizes, endorses, grants, illustrates, implies, insists, notes, observes, points out, reasons, refutes, rejects, reports, responds, suggests, thinks, writes.