I think my top 70s albums are:
1: Big Star: Third
2: Neil Young: Tonight's the Night
3: Ramones: Leave Home
4: Velvet Underground: Loaded
5: Nick Drake: Bryter Layter
On another day, I might have said either of the other two Big Star albums, Zuma or On The Beach by Young, and Pink Moon by Nick Drake.
Honorable mentions to Stooges Raw Power, Suicide's self titled debut, New York Dolls' debut, Kraftwerk's Man Machine, MC5's Back In The USA, John Cale's Paris 1919 and Fear, Joy Division's Unknown Pleasures, Leonard Cohen's Songs of Love and Hate, The Clash's London Calling, Neu and Neu 2, and absolutely loads of other stuff people have already posted here.