I suppose my take on the OP would be along the lines of:
Beatles - Sgt Pepper
Pink Floyd - DSOTM
Rolling Stones - EOMS
The Who - Tommy or the live album
Marvin Gaye - What's Going On?
Beach Boys - Pet Sounds
Led Zep - any of them
Fleetwood Mac - Rumours
Simon & Garfunkle - BOTW
Michael Jackson - Thriller
Or something similar to Road Runner & Alan's lists.
Personally, I own nothing from my list above (& little, if anything, from RR & Alan's lists) and rate DSOM as the most over-rated album I've heard...
My 'classics' list would be along the lines of:
Aphex Twin - Selected Ambient Works 85-92
Boards of Canada - Music Has The Right To Children
Cabaret Voltaire - The Crackdown
Cocteau Twins - Treasure or Victorialand
DJ Shadow - Endtroducing
Kraftwerk - Trans Europe Express
Massive Attack - Mezzanine
Pale Saints - Comforts Of Madness (or My Bloody Valentine - Loveless)
Portishead - Dummy
Shriekback - Jam Science
Stone Roses - Stone Roses
Tricky - Maximquaye
Plus stuff by: The Smiths/New Order/PWEI/Jesus & Mary Chain/Muslimgauze/Godspeed YBE
YMMV by quite a lot