It's very hard to pick just one role from the late Dennis Hopper's enormous body of work, especially since so many are landmark films in their own right. Hopper had smaller roles like Rebel Without a Cause, Giant, and Cool Hand Luke, but even a glimpse of him praying and making the childlike gesture for "Open up the church and see all the people" in Luke makes an indelible impression. He played an innocent sailor in love with a possibly murderous mermaid in Night Tide in 1961, but by 1969 he was already notorious for the tumult surrounding his directorial debut, Easy Rider.
Easy Rider, both the film itself and the history behind the film, symbolized the end of the '60s, the era of peace and love turning dark and violent: the Manson family murders, the Kent State shootings, the Altamont Speedway concert where a Rolling Stones fan was stabbed by one of the Hells Angels' "security guards." However, although Hopper was a co-writer with Peter Fonda and Terry Southern, and a co-star, his presence on-screen is more reactive than active, and for the most part, Peter Fonda and Jack Nicholson were the characters who acted most as catalysts to what plot there was. That statement isn't to dismiss Easy Rider at all, just to point out that it's a road movie, and as such, is fairly meandering and loose.
Naturally, Dennis Hopper's work in Francis Ford Coppola's Apocalypse Now as a photographer lured into Colonel Kurtz's (Marlon Brando) heart of darkness was also a watershed moment in his career. Although he doesn't show up until almost the end of the movie, he acts as a sort of guide to the underworld of Kurtz's death cult for Captain Willard (Martin Sheen), his performance is unforgettable, especially his tripped-out rant about Kurtz's legacy.