Carl Löwe (Loewe) was a real master of the genre, one of the best lieder composers after Schubert (he had also a rather tragic life like him). His most recommended lieder: Edward, Erlkönig, Tom der Reimer and his best cycle Liederkreis nach Gedichten von Friedrich Ruckert.
Mendelssohn also wrote beautuful lieder (Auf flügeln des gesanges is just a little but famous one)
from the late german romantic school :
Richard Strauss : Vier letzte Lieder.
Then Grieg's songs are really superb, most of them I've heard are magnificant ... Especially his famous cycle Op.67 (Haugtussa)
and don't ever forget Debussy and Ravel with their Chansons !
Their best cycles :
Debussy : Ariettes oubliées, Chansons de Bilitis, Fêtes galantes, Ballades de François Villon
Ravel : Histoires naturelles, Poèmes de Stéphane Mallarmé, Chansons madécasses, Don Quichotte à Dulcinée
Delius has also composed many songs, some are really beautiful, inspired by Grieg. Besides, Frank Bridge is an English composer whose songs usually described worthy, but I have not heard a single note from them and can not recommend them personally, as well as my favorite composer Sibelius, I'm sure the great maestro has done it great but can not make any recommendation yet.