I'm voting for the Smith Carbine. Daniel Wesson and Horace Smith became partners before the war, but they didn't manufacture long arms but worked on a repeater handgun. Daniel's older brother under whom he apprenticed, Edwin, made target rifles and one is in the Smithsonian's collection. Edwin passed away before the war.

Check out the book, Carbines of the U.S. Cavalry, 1861-1905 by John D. McAulay, or Joe Bilby's Civil War Firearms or the classic work by William B. Edwards, Civil War Guns.