I couldn't possibly choose just one so below is my list of favourites and albums that have had a major impact on me in one way or another (in no particular order)
Judas Priest - Defenders Of The Faith
Iron Maiden - Seventh Son Of A Seventh Son
Sonata Arctica - Ecliptica
Power Quest - Magic Never Dies
In Flames - The Jester Race
Children Of Bodom - Hatebreeder
Falkenbach - Heralding The Fireblade
Shadow Gallery - Room V
Symphony X - The Divine Wings Of Tragedy
Ayreon - The Human Equation
Nokturnal Mortum - Nechrist
Emperor - In The Nightside Eclipse
Death - Scream Bloody Gore
W.A.S.P - The Crimson Idol
Immortal - At The Heart Of Winter
Evergrey - Recreation Day
Dream Theater - Images And Words
Forefather - Ours Is The Kingdom
Ensiferum - Ensiferum
Bob Catley - When Empires Burn
Magnum - On A Storytellers Night
Amorphis - Am Universum
Therion - Lemuria/Sirius B
Stratovarius - Visions
Rhapsody - Symphony Of Enchanted Lands
Windir - 1184
Okay so that's a shitload of albums but I really feel strongly about all the above and I might have missed a fair few. That's off the top of my head. Quite a diverse list and some bloody great memories I have of some of them.