In terms of "...Mellotron drenched albums..." it depends how drenched they are. In terms of EVERY track having 'tron on it, then it's "Pampered Menial" by Pavlov's Dog - a brilliant album with brilliant vocals (which I know turns the stomachs of at least 2 forum members!)
Love it.
"Starless" (on "Red"), "The Fright Watch" segueing into "The Talking Drum" (on "The Night Watch" album) and "Bartley Butsford" segueing into "Exiles" (on CD3 of the "Great Deceiver" album) by King Crimson. In my view, King Crimson have made the greatest Mellotronic contribution in terms of quantity (over 50 of their albums have 'tron) and quality to this world of Rock that we inhabit.
Concur. They had a musicality that seemed to transcend the need to make the music 'bigger' with a Tron and instead do other things with it instead.
You can also add...
Mike Dickson - "Stroke #2" (on "Six Consequences")
True strory: at least eight notes on that track were played by my (late) cat.