I'll second The Clash - London Calling.

Here's some others I would consider "essential", any combination of the following and yes there's plenty I'm missing. Bold is for emphasis. Without further ado, I present bfx's big and wholly incomplete list of badass records:

The Rolling Stones - Let It Bleed

Pink Floyd - Dark Side of the Moon

The Beatles- Abbey Road

The Beatles - Revolver

Joy Division - Unknown Pleasures or Closer

Neil Young - Everybody Knows this is Nowhere

The Velvet Underground - The Velvet Underground & Nico

Fugazi - Repeater

The Descendents - Milo Goes to College

The Stooges - Raw Power

Husker Du - New Day Rising

Sex Pistols - Never Mind the Bollocks Here's the Sex Pistols

Television - Marquee Moon

The Replacements - Sorry Ma, Forgot to Take out the Trash

Mission of Burma - Vs.

Pavement - Crooked Rain, Crooked Rain

Arcade Fire - Funeral

Boards of Canada - Music Has the Right to Children

Miles Davis - Kind of Blue

Miles Davis - Bitches Brew

Dave Brubeck Quartet - Time Out

Depeche Mode - Music for the Masses

New Order - Substance

Herbie Hancock - Head Hunters

The Band - Music from Big Pink

The Beach Boys - Pet Sounds

Big Star - #1 Record (I don't know how available this one is but it is the essential Power Pop record)

David Bowie - Ziggy Stardust

The Byrds - Sweetheart of the Rodeo

Nirvana - Nevermind

Nick Drake - Pink Moon

Fleetwood Mac - Rumours

Marvin Gaye - What's Going On

Elton John - Goodbye Yellow Brick Road

Van Morrison - Astral Weeks

Prince - Purple Rain

Public Enemy - It Takes a Nation of Millions to Hold Us Back

Radiohead - The Bends

Simon & Garfunkel - Bridge Over Troubled Water

Stevie Wonder - Innervisions