The answer to this question can be found at the very beginning of Mark Twain’s The Adventures of Tom Sawyer. In Chapter 1, Aunt Polly says the thing Tom hates more than anything else is work.
Tom Sawyer is a young boy of about 12 years of age. His parents are both dead and he lives with his Aunt Polly, who is his mother’s sister. At the beginning of the book, we see Aunt Polly looking for Tom. We do not know exactly what she wanted from him, but we find out that Tom has been hiding in a closet, apparently eating jam that Aunt Polly told him not to eat. She is about to whip him with a switch when he tricks her and is able to run away.
After this happens, Aunt Polly thinks to herself about how she needs to try to discipline Tom more effectively. She worries he will not learn how to be a responsible adult. She resolves to catch him the next day and make him work. As she says,
"I'll just be obleeged to make him work, to-morrow, to punish him. It's mighty hard to make him work Saturdays, when all the boys is having holiday, but he hates work more than he hates anything else, and I've got to do some of my duty by him, or I'll be the ruination of the child.”
From this passage, we can see that the thing Tom hates most (at least according to Aunt Polly) is work.