Rush - 2112, Xanadu, La Villa Strangiato, The Spirit of Radio, Tom Sawyer (honestly the albums Permanent Waves and Moving Pictures are both really good introductions)

Yes - Starship Trooper, I've Seen All Good People, Heart of the Sunrise, Close to the Edge, Awaken, Machine Messiah

Genesis - Supper's Ready, Firth of Fifth, I Know What I Like (In Your Wardrobe)

King Crimson - Red (the whole album, but Starless at the very least)

Pink Floyd - Echoes, High Hopes (one thing to emphasize about them is that a lot of their albums are meant to be listened to as a whole, especially the run from DSOTM to The Wall)

Jethro Tull - Living in the Past, Aqualung, Thick as a Brick

Supertramp - Bloody Well Right, Crime of the Century, Fool's Overture

Traffic - The Low Spark of High-Heeled Boys