In which 2017 Marvel Cinematic Universe film does Thor find himself imprisoned on the planet Sakaar and forced to fight the Hulk in a gladiatorial contest?

The 2017 Marvel Cinematic Universe film where Thor finds himself a prisoner on the distant planet Sakaar and is forced to participate in a gladiatorial contest against the Hulk is "Thor: Ragnarok." This movie marked a significant shift in tone for the Thor franchise, largely due to the comedic vision of director Taika Waititi. After being cast out of Asgard by his powerful sister Hela, the goddess of death, Thor tumbles through space and lands on Sakaar, a "landfill of the universe" ruled by the eccentric Grandmaster.

Upon his arrival, Thor is captured and forced into servitude as a gladiator. To his astonishment, his first opponent in the arena is none other than his fellow Avenger, the Hulk, who had disappeared after the events of "Avengers: Age of Ultron" and crash-landed on Sakaar through a wormhole. The Hulk had spent two years on Sakaar as the Grandmaster's beloved champion, and during this time, the Bruce Banner persona had been suppressed, allowing the Hulk to develop a more childlike vocabulary. This unexpected reunion and subsequent battle is a pivotal and memorable sequence in the film, drawing inspiration from the "Planet Hulk" comic book storyline.

"Thor: Ragnarok" is celebrated for its vibrant visuals, humor, and engaging action sequences, with many critics considering it the best installment in the Thor series. The film's lighthearted approach, which allowed for extensive improvisation from the cast, helped to reinvent the character of Thor and infused new energy into the MCU. Beyond the thrilling gladiatorial combat, the film also explores themes of family, destiny, and what it truly means to be a hero, all while maintaining an entertaining and adventurous spirit.