There have been so many style progressions in the 20th century. A few favorites of mine:
-Berg String Qt. and Lyric Suite for String Qt. (by the Alban Berg Qt.)
-Lutoslawski String Qt. (by the Alban Berg Qt)
-for other string quartets contemporary with Bartok but not similar in style, the quartets of Shostakovitch (Emerson Qt) and Zemlinsky (Artis Qt)
And if you like orchestral work,
- Lutoslawski's Mi Parti and Musique Funebre
- most of the orchestral works of Thomas Ades
- the idiosyncratic and non-atonal works of Rautavaara, such as Cantus Arcticus
- John Tavener's Lamentations and Praises, sung by Chanticleer. Tavener has been progressing away from strict minimalism and combining other elements, as Gregm says
-Berg String Qt. and Lyric Suite for String Qt. (by the Alban Berg Qt.)
-Lutoslawski String Qt. (by the Alban Berg Qt)
-for other string quartets contemporary with Bartok but not similar in style, the quartets of Shostakovitch (Emerson Qt) and Zemlinsky (Artis Qt)
And if you like orchestral work,
- Lutoslawski's Mi Parti and Musique Funebre
- most of the orchestral works of Thomas Ades
- the idiosyncratic and non-atonal works of Rautavaara, such as Cantus Arcticus
- John Tavener's Lamentations and Praises, sung by Chanticleer. Tavener has been progressing away from strict minimalism and combining other elements, as Gregm says