THE most sonically dissappointing (lp) you own of great music?

The first couple of Jackson Browne LPs.

Madonna Bed Time Stories. Very very messy bass there. Extremely collectible vinyl tho.
"The Smiths" joins for poor mastering.

I'll tell you what category of LP's are dynamically compressed.  Half-speed masters.  What a total rip-off they are.  When I was young and dumb (I am now old and dumb) I collected these things basically without listening to them, and when my rig improved, I learned just how bad they are.  Except for a few A&M half-speeds, I sold almost all of mine and bought the originals of the titles I wanted.  I also bought a lot of great new hardware with the proceeds.

Brian Ellis Group- Escondido Sessions. Suffers from home brew DIY production but damn! 

Agree with op on War on Drugs

i have yet to pick up an original but I do have an A&M half speed, "audiophile" series of Jobim's Wave that I may have listed as a letdown, it wasn't exactly cheap either, but that wouldn't be totally fair till I hear the original.