Right now, there are a number of prominent conspiracy theories. These range from relatively innocuous conspiracy theories that do not really have a significant impact on the world to major conspiracy theories, such as 911 conspiracy theories, that could impact how people vote, undermine confidence in the government, and have a significant impact on individuals and society at large. To help you choose a conspiracy theory, Wikipedia has a good list of conspiracy theories along with a brief description of the conspiracy.
To write an objective essay, we would encourage you to write about a conspiracy theory that you doubt. Whether or not any conspiracy theories are true or have elements of truth, if you consider yourself a believer, it is much more difficult to objectively discuss the facets of the theory. This bias would make for a lower quality paper.
That does not mean that you cannot think that the theory you discuss has some merit or some reasonable points. In fact, choosing a conspiracy theory that has some elements that you find believable would also make for a more objective paper than choosing a conspiracy theory that you dismiss out of hand. When you write about a conspiracy theory that you find unbelievable, it can be very easy to sound hostile and condescending, which makes your paper seemed bias.
It may seem tempting to choose QAnon as your conspiracy theory because it is dominating much of the news cycle. However, unless you are writing a very lengthy paper, this would be a bad choice. That is because the whole QAnon theory is incredibly complex. To even highlight the main aspects of the theory would take several pages, while an in-depth analysis is more appropriate for a dissertation-length paper.