The Mellotron Group

King Crimson have (in the 1969-1974 period) produced the most and best 
music with Mellotron. Out of their repertoire, I'd pick...
Epitaph (ITCOTCK)
In the Wake of Poseidon (ITWOP)
Sailor's Tale (Islands)
Starless (Red)
The Fright Watch & The Talking Drum (The Night Watch)
Barley Butsford & Exiles (CD3 on The Great Deceiver)

Outside Crimson, then its Pampered Menial by Pavlov's Dog. EVERY track 
has Mellotron, and it's quality. Check out the vocals "...like a choir 
boy on speed..." There are many others I could choose, but a posting 
can only be so big!

Nick, fae South Embra