Less Than Zero (1987)
Ellis never quite liked the adaptation of his first novel. While Harley Peyton's screenplay does a decent enough job of capturing the casual decadence of the source text, Ellis and many critics have pointed out how poorly cast Less Than Zero is. With the exception of Robert Downey Jr. and James Spader as a drug addict and his dealer, respectively, the film suffers from being populated with actors chosen for their popularity rather than talent. Andrew McCarthy doesn't really have the depth or charisma to play Ellis's disaffected stand-in Clay, while Jami Gertz doesn't carry the role of his messed up love interest Blair with near as much complexity as it requires. A bit of a stuttering start, Less Than Zero tries a bit too hard to be hip, forgetting that the story is all about how pointless and destructive youth trends can be.
American Psycho (2000)
Ellis wouldn't see another one of his stories adapted for the screen until Mary Harron's chilling take on America Psycho. A perfectly cast Christian Bale plays Patrick Bateman, a Manhattan executive who may or may not be a brutal murderer. The novel itself is nearly unadaptable with its long, ironic treatises on pop culture and its sickeningly detailed scenes of violence. Harron pulls back from Ellis's satire of 80's corporate culture and focuses on a character study of Bateman himself. The slick, cutthroat world he occupies looks startlingly similar to the Bubble Era prosperity in which the film was made. If Harron's Bateman had gone on a long defense of Ricky Martin rather than Huey Lewis just before killing his coworker with an axe, the scene would have come out just the same.
The Rules of Attraction (2002)
Roger Avary's subtly iconoclastic adaptation of Ellis's 1987 follow-up to Less Than Zero is perhaps the most loyal to Ellis's style and sensibility. The Rules novel is a series of disjointed narratives from various individuals at the fictional Camden College that, together, capture the confusion and hedonism of college culture. Avary's adaptation is more of a linear, ensemble story but it conveys all of those same ideas with a fair amount of style. It's ostensibly set in the year it was made, though the isolation and dark absurdity of the characters' lives could really take place any time in the past few decades. Rules of Attraction is a dark twist on the usual college movie tropes. Its parties are uglier, its indulgences more dangerous and its romances considerably less rose-tinted.
The Informers (2009)
This most recent Ellis adaptation hasn't fared well with critics or viewers. While the short story collection on which it is based has been taken as a decent intro to Ellis's style, Gregor Jordan's adaptation has been more or less disowned by both Ellis and his fans as a project no less vapid than the characters it depicts. Far from essential, The Informers is perhaps best seen as a sign of how the same sentiment can only be expressed so many times before it falls flat on execution.
Extra Credit: This is Not an Exit (2000)
Not exactly an adaptation and not exactly a biography, Gerald Fox's super-indie film This is Not an Exit deconstructs both Ellis and his characters on a shoestring budget with a very, very limited release. It's hard to find and it's not entirely kind to its subject, either. It suggests that Ellis's writing is inherently naive, however interesting. If you can get your hands on a copy, it's worth watching.