First and foremost, anything by Fromental Halévy (1799-1862) that isn’t La Juive.
Halévy was the leader of the French school of opera composers; he was praised for his profound science (sometimes too advanced for contemporaries used to simpler stuff), his dramatic understanding, and his penetrating melancholy.
At the very least L’Eclair (1835) (a musical tour de force with only four singers and no chorus, written to prove that the success of La Juive didn’t rely on spectacle) and the three opéras-comiques of the late 1840s, Les Mousquetaires de la Reine (1846), Le val d’Andorre (1848), and La fée aux roses (1849), three of the biggest successes in that theatre’s history.
As to grand opéra: we need a new recording of Charles VI (1843) (Bru Zane might oblige?). Guido et Ginevra (1838) – set in Renaissance Florence during the Plague – has a superb tomb scene, but suffers from a superfluous last act. Opera Rara will record this with Michael Spyres.
Perhaps also Le Shérif (1839) (Berlioz: ““Never has M. Halévy been so abundant, so rich, and above all so original… I experienced almost from start to finish that rare pleasure that bold, novel, and skilfully co-ordinated compositions give to musicians.”); Le Guitarrero (1841); and La dame de pique (1850), a clever plot whose score has been called a magnificent jewel case.
Apart from Halévy…
Jules Massenet‘s Bacchus (1909).
Méhul‘s Euphrosine (1790).
Jean Nouguès’s Quo vadis? (1909).
Giovanni Pacini‘s Lorenzino de’ Medici (1858): “a superb opera by Pacini, and one that for a time made me stagger in my Verdi faith. It is so fresh, so original, and combines musical science so well with ear-haunting and simple melody that it appears to me astonishing that it has not obtained a reputation out of Italy.” (Dwight’s Journal of Music)
and Niccolò de’ Lapi (1873): “Meyerbeer and Wagner and the Verdi of Forza del Destino, of Don Carlos, of Aida, have found a powerful rival, a true titan, in the immense and stupendous finale of the second act.” (La Riforma)
Ernest Reyer‘s La statue (1861): Bizet thought it the most remarkable work written in France for twenty or thirty years. Massenet, who played the timpani in the orchestra, called it a superb score. Berlioz found it moving; the melody was original, witty and natural, the harmony was colourful, and the instrumentation was energetic without brutality or violence.
Gaston Salvayre’s La dame de Monsoreau (1888).