I first heard Ms. Hunt-Lieberson singing the "Urlicht" movement from Mahler 2 (SF Symphony/Tilson Thomas) which completely blew me away, and more recently, alongside another favorite of mine, Dawn Upshaw, singing (my first music composition teacher) John Adams' piece, "El Nino".
For me, Lieberson was one of these rare "opera singers" who didn't sound like most other opera singers -- No stupid vibrato and crazy over-emphasis of technique; none of the "I'm a great singer" crap to be heard anywhere at anytime. Her voice seemed natural; more emotional than technical to me, (though her technique, as far as I can tell, was exceptional.) She is one of a few singers who can melt my heart to a puddle of red metalic ooze in a phrase or two.
Prior to hearing her, I'd always been partial to Janet Baker. After my experience with the Mahler 2, I just figured I'd be buying any new versions of Mahler Song Cycles that Lieberson would presumably be recording. (I was especially looking forward to "Ich Bin der Welt Abhanden Gekommen" from Mahler's Ruckertlieder)
Sadly, this won't happen; Lorraine Hunt Lieberson passed away on July 3rd. ...sigh