Pärt is, in my opinion, really amazing with 'De profundis' and 'Berliner Messe'.
The 'Sieben Magnificat-Antiphonen' is, as someone earlier mentioned, most definitely enjoyable.
My favorites, in no specific order:
Arvo Pärt - Berliner Messe + De profundis
Palestrina - Missa aeterna christi munera + Sicut cervus
Morten Lauridsen - Lux aeterna (especially 'O nata lux')
Steve Dobrogosz - Mass
Nobuo Uematsu - Song of Prayer (variations)
Eric Whitacre - Sleep + Lux aurumque
Gabriel Fauré - Requiem, especially 'Pie Jesu'
Frank Martin - Messe pour double choeur a cappella
Ingvar Lidholm - Laudi
Maurice Duruflé - Requiem + Ubi caritas et amor (from Quatre motets)
Händel - Zadock the Priest
Felix Mendelsohn - Hear My Prayer (the soprano solo is stunning)