My gosh, I have lot
JFK for me is the best. Costner, Tommy Lee Jones, Kevin Bacon, Gary Oldman, Sissy Spacek, Joe Pesci, Jack Lemmon, Walter Matthou, Ed Asner, Donald Sutheland, Wayne Knight, Vincent D'onoforio, Laurie Metcalf and John Candy. Hard to find a more prestigious cast.
Honorary Mentions:
Heat - In addition to Pacino and Deniro sharing the screen for the first time, you get John Voigt, Tom Sizemore, Ashley Judd, Val Kilmer, Hank Azaria, Ted Levine, Dennis Haysbert, Amy Brenneman William Fichtner and Natalie Portman all providing support.
It's A Mad, Mad, Mad, Mad World - It's quite simply the best cast ever assembled for a comedy. Some of the greatest legends in the comedic field from Milton Berle to The Three Stooges to Buddy Hackett to Buster Keaton to Jimmy Durante (plus over 40 more, by my count) all in the same film is a joy to watch.
Pulp Fiction - a cliched choice, but the ensemble is impressive nonetheless. Not to mention that John Travolta, Sam Jackson, Uma Thurman and arguably Bruce Willis all turn in their finest performances here, and are backed a heck of a group of supporting players.
The Outsiders - great assembly by Francis Ford Coppolla of many 1980s' icons. Patrick Swayze, Emilio Estevez, Matt Dillon, Rob Lowe, Ralph Maccio, Diane Lane and C. Thomas Howell arguably the finest perfromances of their careers. Heck, even Tom Cruise turns in a solid supporting role.
Almost Famous - My favorite movie of all time, and a great cast to boot. Virtually no A - listers, but a solid gathering of some of the most underrated actors in Hollywood. In addition to Kate Hudson giving one of the greatest performances of all time, Patrick Fugit, the amazing Billy Crudup, Jason Lee, Philip Seymour Hoffman, Anna Paquin, Fairuza Balk, and Frances McDormand all provide a solid supporting cast.
Glengary Glenross - I'll admit, I've yet to see the movie, but the cast does peak my interest. Just the sheer thrill of seeing Al Pacino and Jack Lemmon share the screen is enough for me, but to be backed up by the likes of Kevin Spacey, Alan Arkin, Ed Harris and Alec Baldwin in arguably his finest hour just adds to it. Great cast all the way.
True Romance - Truly one of the best. Christian Slater and Patricia Arquette turn in their finest performances, and are backed up by the likes of Dennis Hopper, Gary Oldman, Christopher Walken, Val Kilmer, James Gandolfini, Tom Sizemore, Chris Penn, Michael Rapaport, Sam Jackson, and Balky from Perfect Strangers.
The Departed - a fantastic cast assmebled by quite possibly the finest director of the last 30 years. Leo DiCaprio cements himself as one the finest actor working today, and Matt Damon and Mark Wahlberg also prove their worth to a great degree. Add a supporting cast that includes veterans Martin Sheen, Alec Baldwin and of course, Jack Nicholson, and you get to date the best mainstream ensemble of the 21st century.