Hmmm....tough to pick just one. Despite its short duration, I would probably nominate Ravel's Pavane pour une infante défunte (the piano version was written in 1899, but the orchestral version comes from 1910, so...). Such a beautiful piece! VW's Tallis Fantasia and Symphony no. 5, Barber's Adagio for Strings (overused though it be), Sibelius' Symphony no. 5, Larsson's Symphony no. 1, Atterberg's Symphony no. 3 and (more recently) Aaron Jay Kernis' Musica celestis for strings would also be prime candidates. Oh, and then there's Joep Franssens' orchestral works!
Larsson no 3 gets my vote..