Here some of my favourite character roles. I decided to mention just one film per actor, but honestly, most of these actors have played more than one great role, so this was just the pick of the day. Still, the Henry Fonda role is surely a top 3 one, and the Anthony Quinn role would be in my top 5 as well. The three Japanese guys I mention are sublime actors, who made many films I would recommend to anyone (especially the ones in the 1950s and 1960s, directed by Akira Kurosawa and Masaki Kobayashi).
Juror #8 by Henry Fonda in 
12 Angry Men (1957) 
Col. Andrea Stavros by Anthony Quinn in 
The Guns of Navarone (1961)
Kôichi Nishi by Toshirô Mifune in 
The Bad Sleep Well (1963)
Kambei Shimada by Takashi Shimura in 
Seven Samurai (1954)
Kaji by Tatsya Nakadai in 
The Human Condition (trilogy) (1959, 1959, 1961) 
Lt. Commander Phillip Queeg by Humphrey Bogart in 
The Caine Mutiny (1954)
Terry and Ruth Collins by Olivia de Havilland in 
The Dark Mirror (1946)
A family ancestor in a portrait, Young Ascoyne D'Ascoyne (son of Lord Ascoyne D'Ascoyne), Young Henry D'Ascoyne (photographer), Reverend Lord Henry D'Ascoyne, Lady Agatha D'Ascoyne, Admiral Lord Horatio D'Ascoyne, General Lord Rufus D'Ascoyne, Ethelred D'Ascoyne and 8th Duke of Chalfont Lord Ascoyne D'Ascoyne (Banker, 9th Duke of Chalfont) 
 by Alec Guinness in 
Kind Hearts and Coronets (1949) 
Dr. Henryk Savaard by Boris Karloff in 
The Man They Could Not Hang (1939)
Roger Thornhill by Cary Grant in 
North by Northwest (1959)
Richard Dadier by Glenn Ford in 
Blackboard Jungle (1955)
Atticus Finch by Gregory Peck in 
To Kill a Mockingbird (1962)
Dr. Luther Brooks by Sidney Poitiers in 
No Way Out (1950)
John J. Macreedy by Spencer Tracy in 
Bad Day at Black Rock (1955)
Jefferson Smith by James Stewart in 
Mr. Smith Goes to Washington (1939)
Mr. Slade, the lodger by Laird Cregar in 
The Lodger (1944)
Sherlock Holmes by Basil Rathbone in 
The Scarlet Claw (1944)
Victor Grandison by Claude Rains in 
The Unsuspected (1947)
Otto Becker / Baron Hugo Von Detner by Conrad Veidt in 
Nazi Agent (1942)
Alicia Huberman by Ingrid Bergman in 
Notorious (1946)
Eric Gorman by Lionel Atwill in 
Murders in the Zoo (1932) 
Louise Howell by Joan Crawford in 
Possessed (1947)
Col. Dax by Kirk Douglas in 
Paths of Glory (1957)
Ben Stride by Randolph Scott in 
Seven Men from Now (1956)
Gustave 'Gu' Minda by Lino Ventura in 
Le deuxième souffle (1966)
Marshal Will Kane by Gary Cooper in 
High Noon (1952)
Professor Richard Wanley by Edward G. Robinson in 
The Woman in the Window (1944)
Sefton by William Holden in 
Stalag 17 (1953)