It's hard to say just one, so here's two each
Pink Floyd - Dark Side Of The Moon, A Saucerful Of Secrets
Yes - Close To The Edge, 90125
King Crimson - In The Court Of The Crimson King, Larks' Tongues In Aspic
Rush - Moving Pictures, 2112
Genesis - Duke, A Trick Of The Tail
Since you already have two of those, it would seem you're on your way to awesome.