I've decided to go through a bunch of classic film comedians state who I think is either their most iconic or greatest supporting actor. Feel free to disagree or discuss or any other ones you think are worthy.
Mabel Normand: Charlie Chaplin (technically Mabel was supposed to be the star in the films, they had her name in the title after all)
Roscoe Arbucke: Buster Keaton
Charlie Chaplin: Eric Campbell (honorable mention to Paulette Goddard)
W.C. Fields: Kathleen Howard
Abbott and Costello: Sidney Fields
Harry Langdon: Vernon Dent
Buster Keaton: Jimmy Durante (Probably not necessarily his best. I just don't remember his supporting casts too well, but I certainly remember Durante in his movies)
Harold Lloyd: Jobyna Ralston
Marx Brothers: Margaret Dumont (obviously)
Bob Hope: Dorothy Lamour
The Three Stooges: Emil Sitka (I probably would have stated Vernon Dent, but I put him with Harry Langdon instead. Emil was also the only actor to work with all four of the third stooges, and a definitely honorable mention to Christine McIntyre).
Charley Chase: Thelma Todd
Laurel and Hardy: James Finlayson (honorable mentions to Anita Garvin and Mae Busch)
Our Gang: June Marlowe as Miss Crabtree
Wheeler and Woolsey: Dorothy Lee
Harold Lloyd: Jobyna Ralston
W.C. Fields: Kathleen Howard
Abbott and Costello: Sidney Fields
Harry Langdon: Vernon Dent
Buster Keaton: Joe Roberts
Charlie Chaplin: Eric Campbell
Roscoe Arbuckle: Buster Keaton
Laurel and Hardy: James Finlayson - though too many tough choices here
Marx Brothers: Zeppo....uh, I mean Margaret Dumont
Wheeler and Woolsey: Dorothy Lee
Mabel Normand: Roscoe Arbuckle, who she had way more chemistry on screen chemistry with than Chaplin, off screen may be another story
Three Stooges: Vernon Dent - Sitka was great but also appeared with Curly for a cup of coffee.
Bob Hope - Dorothy Lamour