However, if I had to choose, it would likely be Harris.
Here's why:
* He's incompetent: Harris is constantly making mistakes and causing chaos. He can't seem to do anything right, from packing the boat to cooking meals.
* He's self-centered: Harris is often focused on his own comfort and enjoyment, even at the expense of his companions. He's also prone to making impulsive decisions that inconvenience others.
* He's a bit of a bully: While it's played for laughs, Harris is often sarcastic and dismissive of Montresor and George's ideas.
Ultimately, Harris's incompetence and self-centeredness are what make him the most frustrating character. He's the source of much of the chaos and aggravation that the trio faces on their journey.
However, it's important to remember that *Three Men in a Boat* is a comic novel, and Harris's flaws are meant to be humorous. He's also capable of kindness and generosity, and the novel wouldn't be the same without his chaotic presence.