The four-part story arc that comprises Destination Moon and Explorers on the Moon, because of its complex plot and all the action and danger, especially when it's revealed just what Professor Calculus can do.
In "The Blue Lotus", three giant brute cops get sent into Tintin's jail cell to "teach him a lesson". One panel later we see the three cops in hospital beds, severely injured.
Captain Haddock chasing off the parasites in Red Rackham's Treasure by announcing he feels his ancestor's spirit being channelled through him.
The Crab With The Golden Claws: Captain Haddock charging the Bedouin, screaming vengeance for his shattered whiskey bottle as the Bedouin flee. (Yes, it was because the cavalry was coming in behind, but he was willing to run up to men with guns, insulting them all the while.)
All of Tintin in Tibet, really. Tintin's determination to find Tchang and Haddock's incredible loyalty.
Haddock's reaction to finding out about the slave smuggling in The Red Sea Sharks. His list of insults towards the bad guys never seems to end.
In a non-action example, Bianca Castafiore slamming the doors of Marlinspike Hall in Jolyion Wagg's face in The Castafiore Emerald is an extremely satisfying and long-overdue moment.
Castafiore gets another one in The Calculus Affair where her ability to improvise saves Tintin and Haddock from capture and provides them with the means to defeat the Big Bad and escape Borduria.
On driving a stolen tank to the Bordurian border, Tintin finds the road blocked by heavy trucks and the border protected by anti-tank obstacles. So he drives the tank straight through the guardhouse.
The story about how King Ottokar of Syldavia gave his sceptre its mythical status by beating off a violent would-be usurper with it; it helps that his Bond One-Liner from the incident ("If you gather thistles, expect prickles") became the Syldavian national motto.