Most blatent song quotations ever...

Elastica, for butchering Wire's Three Girl Rhumba [Pink Flag] in their hit Connection. They settled out of court.

Closely followed by Devo's Uncontrollable Urge [Q: Are we not men? A: We Are Devo!] which liberally lifts from Led Zeppelin's Misty Mountain Hop [Led Zeppelin IV] in its intro.

NOTE: Least blatent song quotation ever - Neil Young's Borrowed Tune [Tonight's the Night] which he 'borrowed' from the Rolling Stones' Lady Jane [Aftermath]. Sing to the tune of Lady Jane:

I'm singin' this borrowed tune / I took from the Rolling Stones
Alone in this empty room / Too wasted to write my own

The implications of this lyric are massive and bleak. Massively bleak, even, possibly.

Others, perhaps?

EDIT: Allmusic.com's Stephen Thomas Erlewine comes up with some bullshit argument in support of Elastica's 'song-writing' practices. I also overhauled the post, re-doing all the links that mysteriously vanished from the published version.