Orchestral masterpieces featuring the contrabass
The oldest orchestral piece featuring an impressive contrabass solo is likely Haydn's Symphony No. 6 in D major ("Morning"). The third movement of this symphony, Menuet e Trio, features a section where the contrabass carries the main melody. Mozart's "Per Questa Bella Mano" (K612), a concert aria composed in Vienna in 1791 for a bass vocalist, also includes a contrabass solo part that requires a truly skilled player.
Although not a symphony, in the 19th century we have "The Elephant" from "The Carnival of the Animals," composed by C. Saint-Saens (1835-1921). Saint-Saens used a galloping melody (which also appears in the overture to J. Offenbach's comic opera, "Orpheus in the Underworld") played on a contrabass to represent a massive elephant dancing. As a piece of music it overflows with a sense of humor.
The main theme of the third movement of G. Mahler's "Symphony No. 1 in D Major" ("Titan") is first presented by a solo contrabass. It's said that this movement was written to depict a funeral march, and the solemn reverberation of the contrabass perfectly brings to mind a sort of slow movement.