This was not easy. I had a particularly difficult time with the 50s and 60s, since I have so many favorites from those decades. But I limited it to 5 per decade, so that's what I stuck to. A bit of a cheat in the 1920s because I believe Chaplin wrote those scores in a later decade. But the other silent scores listed were written at the time. These are my favorites, not necessary the greatest, but in many cases I think they are considered the greatest.
1920s The Kid - Charlie Chaplin Nosferatu - Hans Erdmann Nibelungen - Gottfried Huppertz Metropolis - Gottfried Huppertz October (Ten Days That Shocked the World) - Dmitri Shostakovich
1930s King Kong - Max Steiner Bride of Frankenstein - Franz Waxman The Man Who Could Work Miracles - Mischa Spoliansky Things to Come - Sir Arthur Bliss Alexander Nevsky - Sergei Prokofiev
1940s Citizen Kane - Bernard Herrmann The Best Years of Our Lives - Hugo Friedhofer Treasure of the Sierra Madre - Max Steiner Hamlet - Sir William Walton The Third Man - Anton Karas
1950s The Thing - Dimitri Tiomkin Vertigo - Bernard Herrmann Boy on a Dolphin - Hugo Friedhofer An Invention for Destruction (The Fabulous World of Jules Verne) - Zdenek Liska Ben-Hur - Miklos Rozsa
1960s Spartacus - Alex North Mutiny on the Bounty - Bronislau Kaper 4 Horsemen of the Apocalypse - Andre Previn Cleopatra - Alex North The 10th Victim - Piero Piccioni
1970s The Godfather - Nino Rota Chinatown - Jerry Goldsmith The Taking of Pelham 1-2-3 - David Shire Tess - Philippe Sarde Superman - John Williams
1980s Dragonslayer - Alex North Koyannisquatsi - Philip Glass Body Heat - John Barry Under Fire - Jerry Goldsmith Frantic - Ennio Morricone
1990s Jennifer 8 - Christopher Young Othello - Charlie Mole Cleopatra - Trevor Jones The Joy Luck Club - Rachel Portman Magnolia - Jon Brion
2000s Gladiator - Hans Zimmer and Lisa Gerrard The Usual Suspects - John Ottman The Time Machine - Klaus Badelt Casino Royale - David Arnold The Painted Veil - Alexandre Desplat
2010s The Ghost Writer - Alexandre Desplat Let Me In - Michael Giacchino Sherlock Holmes - Hans Zimmer The Wolfman - Danny Elfman Man of Steel - Hans Zimmer
Interesting variety of choices. I'm not familiar enough with pre-'60s film music, so I'll begin my favorites in the latter half of the century.
1960s House of Usher - Les Baxter Contempt - Georges Delerue The Good, the Bad and the Ugly - Ennio Morricone You Only Live Twice - John Barry On Her Majesty's Secret Service - John Barry
1970s Diamonds Are Forever - John Barry Assault on Precinct 13 - John Carpenter Superman - John Williams Alien - Jerry Goldsmith Moonraker - John Barry
1980s Blade Runner - Vangelis Conan the Barbarian - Basil Poledouris The Secret of NIMH - Jerry Goldsmith A View to a Kill - John Barry Batman - Danny Elfman
1990s Alien 3 - Elliot Goldenthal Jurassic Park - John Williams Black Beauty - Danny Elfman Independence Day - David Arnold Starship Troopers - Basil Poledouris
2000s Atlantis: The Lost Empire - James Newton Howard Final Fantasy: The Spirits Within - Elliot Goldenthal The Lord of the Rings: The Return of the King - Howard Shore The Matrix Reloaded - Don Davis Batman Begins - Hans Zimmer and James Newton Howard
We're only halfway through the current decade so I'll hold off on my top 5 for the 2010s for now, but I'm sure in five years' time my list will include TRON: Legacy by Daft Punk and How to Train Your Dragon (1 and/or 2) by John Powell.
2000s Gladiator - Hans Zimmer and Lisa Gerrard The Usual Suspects - John Ottman The Time Machine - Klaus Badelt Casino Royale - David Arnold The Painted Veil - Alexandre Desplat
The Day the Earth Stood Still (Herrmann) The Ten Commandments (Bernstein) The 7th Voyage of Sinbad (Herrmann) Ben-Hur (Rozsa)
1960s
The Time Machine (Garcia) Mysterious Island (Herrmann) Jason and the Argonauts (Herrmann) Planet of the Apes (Goldsmith) True Grit (Bernstein)
1970s
Dirty Harry (Schifrin) King Kong (Barry) Star Wars (Williams) Superman (Williams) Star Trek TMP (Goldsmith)
1980s
Raiders of the Lost Ark (Williams) Conan the Barbarian (Poledouris) The Challenge (Goldsmith) Indiana Jones and the Temple of Doom (Williams) Back to the Future (Silvestri)
1990s
Total Recall (Goldsmith) Dances With Wolves (Barry) Far and Away (Williams) First Knight (Goldsmith) Starship Troopers (Poledouris)
2000s
Gladiator (Zimmer) The Fellowship of the Ring (Shore) Signs (Howard) The Incredibles (Giacchino) Revenge of the Sith (Williams)
1930's King Kong - Max Steiner Adventures of Robin Hood - Erich W Korngold Bride of Frankenstein - Franz Waxman Gone With the Wind - Max Steiner Things to Come - Arthur Bliss
1940's Sea Hawk - Erich W Korngold Hangover Square - Bernard Herrmann Citizen Kane - Bernard Herrmann Ghost and Mrs Muir - Bernard Herrmann Devil and Danier Webster - Bernard Herrmann
1950's The Day the Earth Stood Still - Bernard Herrmann Ben Hur - Miklos Rozsa Vertigo - Bernard Hermann The 7th Voyage of Sinbad - Bernard Herrmann North by Northwest - Bernard Herrmann
1960's Lawrence of Arabia - Maurice Jarre Spartacus - Alex North Planet of the Apes - Jerry Goldsmith Magnificent Seven - Elmer Bernstein Psycho - Bernard Herrmann
1970's Star Wars - John Williams Star Trek: The Motion Picture - Jerry Goldsmith The Omen - Jerry Goldsmith Jaws - John Williams Alien - Jerry Goldsmith
1980's Back to the Future - Alan Silvestri Hoosiers - Jerry Goldsmith Raiders of the Lost Ark - John Williams E.T. - John Williams Batman - Danny Elfman
1990's Edward Scissorhands - Danny Elfman Hook - John Williams Jurassic Park - John Williams Black Beauty - Danny Elfman The Ghost and the Darkness - Jerry Goldsmith
2000's Lord of the Rings (all of them or Return of the King to pick one) - Howard Shore Memoirs of a Geisha - John Willaims Chicken Run - John Powell/Harry Gregson Williams The Aviator - Howard Shore The Incredibles - Michael Giacchino
2010's John Carter - Michael Giacchino How to Train Your Dragon (&2) - John Powell The Adventures of Tin Tin - John Williams Alice in Wonderland - Danny Elfman Hugo - Howard Shore