I agree Dies Irae - the words themselves are terrifying! Mozart's setting is superbly dramatic, but the plainsong original is pretty scary too:
Similarly William Byrd's 'Ne irascaris Domine' ('Don't be angry, Lord'), especially the last part, a despairing cry from utter desolation:
Thomas Tallis's Lamentations of Jeremiah, and the Allegri Miserere are in the same league.
Schubert's Death & the Maiden, of course, ever since Ariel Dorfmann used that for his play (later film) - also Schubert's C Major string quintet, that Eichmann puts on to play on the gramophone at the end of the Wansee Conference when the 'Final Solution' was agreed, in the film 'Conspiracy':
And the whole of Winterreise.
'Non, je ne regrette rien' I've requested for my funeral, whenever that might be!