Rubber Soul - The Beatles
To me this marks a turning point in the Beatles' output, from fun, rock 'n' roll/pop music, to serious artistry, more challenging themes and lyrics and more interesting instrumentation.

Sample track: In My Life

Dubnobasswithmyheadman - Underworld
Hugely important in British dance music, a total departure from their first two albums and the start of a run of classic electronic music. It's a shame they're still best known for Born Slippy, because there's so much more to Underworld than that, and it started here.

Sample track: Dirty Epic

The Velvet Underground and Nico - The Velvet Underground
Famously known as an album that not many people bought, but all of those who did started a band. Hugely influential, full of great songs, some gentle and fragile, others cacophonic and dissonant. A masterpiece.

Sample track: Venus in Furs