I would make several 80-minute CD's, including:
Best of Growly Opeth 1 (Blackwater Park & Deliverance)
01. The Leper Affinity (edited)
02. Bleak (edited)
03. The Drapery Falls
04. Blackwater Park
05. Deliverance
06. Master's Apprentices
07. By the Pain I See in Others (edited)
Best of Growly Opeth 2 (Ghost Reveries & Watershed)
01. Ghost of Perdition
02. The Baying of the Hounds
03. Beneath the Mire
04. Harlequin Forest (edited, minus Reverie)
05. The Grand Conjuration
06. Heir Apparent
07. The Lotus Eater (edited)
08. Hessian Peel (edited)
Best of Opeth's Top 40 Hits (Damnation & Other Stuff)
01. Windowpane
02. In My Time of Need
03. Death Whispered a Lullaby
04. Closure
05. Hope Leaves
06. To Rid the Disease
07. Ending Credits
08. Weakness
09. Hours of Wealth
10. A Fair Judgment (edited)
11. Burden (edited)
12. Porcelain Heart
13. Hex Omega
Orchid
Morningrise
Still Life
This works for me, as I have a 6-disc CD player in my car.
I was going to make a "Best of Old Opeth (Orchid & Morningrise)" compilation, but there's too many good tunes on those albums to fit onto one 80-minute CD. I also really really dig their older stuff.
Also, I never really liked MAYH that much, so nothing made the cut. Nope, not even Demon. The "wall of sound" style of the album turns me off.
However, if it's in the fall or winter and I really want to be in the "Opeth mood," I'll just listen to the full album (or "observation").