That's a tough one. Metallica's Ride the Lightning and Master of Puppets are definitely up there. Dream Theater's Scenes From a Memory is nearly perfect. Opeth's Blackwater Park is also nearly perfect, and Old Crow Medicine Show's Big Iron World is great for when I want to listen something a bit less complex but more fun. Lately, I've been listening to Animals as Leaders, The Devin Townsend Project and Scale the Summit, and AAL and DTP's Ki and Addicted are absolutely mindblowing, so right now they are at the top of the list. But the ones I mentioned above are the all-time greats for me. Any one of them, depending on my mood.