That's a really hard question, because "The Adventures of Tom Sawyer" is so full of interesting incidents. It is hard to just pick one. I have always liked the opening fence painting sequence though.
The previous day Tom skipped school, got in a fight, and came home really dirty. As a punishment Tom is made to whitewash a fence. He is sorely disappointed at his punishment, but he turns it around in his favor. Boys come along to tease him about having to paint the fence, but Tom makes them think they are missing out on something by not painting the fence. Tom convinces boy after boy to trade some treasure of theirs for the privilege of painting the fence.
Tom gave up the brush with reluctance in his face, but alacrity in his heart. And while the late steamer Big Missouri worked and sweated in the sun, the retired artist sat on a barrel in the shade close by, dangled his legs, munched his apple, and planned the slaughter of more innocents. There was no lack of material; boys happened along every little while; they came to jeer, but remained to whitewash.
It's a great sequence because it shows how powerfully influential Tom is and how entrepreneurial he can be.