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.
But it wasn't really until he joined forces with David Lynch for Blue Velvet that Hopper got a starring role to make his own. Although Hopper worked steadily his entire life, Blue Velvet brought him back to the forefront of independent cinema. His character Frank Booth is one of the most famous bad guys in film, and a role he demanded from Lynch. The director was hesitant at first because of Hopper's reputation, but his then-recent work on another movie had gone smoothly, so Hopper was hired. Lynch said in an interview in the book Lynch on Lynch, "Dennis was Frank, but luckily he was also someone else too, so it worked out extremely well." (Rodley, Chris, ed., Lynch on Lynch, Faber and Faber, 2005, p. 142.) I'm assuming that the film Lynch refers to is River's Edge, another favorite Hopper role of mine; Hopper plays the desperate Feck, whose only companion is a blow-up doll and the teens who come to visit him for his really good weed and advice on how to deal with a dead schoolmate.
No one else could have played Frank Booth, the psychopathic mobster whose sexual proclivities could have given Krafft-Ebing fodder for a sequel to Psychopathia Sexualis. A twisted, sadistic, gas-huffing freak, Booth tortures the gorgeous siren Dorothy Vallens (Isabella Rossellini) mentally and physically, hitting her, raping her, choking her with blue velvet, and threatening her life and the lives of the ones she loves. He plays psycho sex games in "the dark" with Vallens; first he's the daddy, then he's the baby, and she's the mommy. Hopper owns every scene he's in, and no matter how bizarre or foul Booth's actions are, Hopper dives in without blinking. It's not just David Lynch's crazy screenplay that made Booth's words a part of the cultural lexicon, but Hopper himself.
"Why are there people like Frank? Why is there so much trouble in this world?" asks naïve young Jeffrey Beaumont (Kyle MacLachlan), who stumbles into the underbelly of his small town by finding a severed ear. That question goes unanswered, because in Blue Velvet, Frank Booth is this sleepy town's heart of darkness.