As much which composers as which works, but some Stravinsky, Bartok, Janacek should be in there. Also Shostakovich – all with pretty well unique sound signatures (listen to some and see what I mean).
So, the likes of Stravinsky Firebird, Pulicinella; Bartok Romanian Dances, concertos; Janacek – Glagolitic Mass, string quartets.
Brahms – clarinet quintet.
Byrd – choral works, see also Carver & Tallis
Much more modern – Steve Reich?
Middle distance – some of the Britten works are sublime, may be a bit long or intricate at first though.
I explored quite a bit of classical music by just going to concerts and listening to Radio 3 – get the education bit as well – and also having listened to the likes of E L & P, followed up what they based their works on. Hence the Janacek, Bartok above.