My choices span many genres, and are not all necessarily stuff I listen to on a regular basis - but they changed my perceptions of what is possible in music, or are what I consider groundbreaking, pioneering, the epitome of their form, extremely powerful / compelling / inspiring, or all of the above.
In no particular order here are some that come to mind (will possibly add more)...
Nick Drake - everything
Current 93 - Thunder Perfect Mind, Tamlin EP, Lucifer over London EP
Comus - First Utterance
In Gowan Ring - Twin Trees, the Glinting Spade
Shirley Collins - The Power of the True Love Knot
Donovan - H.M.S. Donovan
Vashti Bunyan - Just Another Diamond Day
Incredible String Band - The Hangman's Beautiful Daughter
C.O.B. (Clives Original Band) - Moyshe McStiff and the Tartan Lancers of the Sacred Heart
Stone Breath - A Silver Thread to Weave the Seasons
Black Sabbath - self titled, Paranoid
Bathory - Hammerheart
Morbid Angel - Altars of Madness
My Dying Bride - Turn Loose the Swans, The Angel and the Dark River
Godflesh - Streetcleaner
Blind Guardian - Imaginations From the Other Side, Nightfall in Middle Earth
Demons and Wizards - self-titled
Fates Warning - The Spectre Within, Awaken the Guardian
Agalloch - The Mantle
Bal-Sagoth - Starfire Burning Upon the Ice-Veiled Throne of Ultima Thule
Burzum - Hvis Lyset Tar Oss, Filosofem
Death - Sound of Perseverance
Cynic - Focus
Gorguts - Obscura
Ulver - Bergtatt, Nattens Madrigal
Dissection - Storm of the Light's Bane
Blut Aus Nord - The Work Which Transforms God
Weakling - Dead as Dreams
Elend - Les Ténèbres du Dehors
Kyuss - Welcome To Sky Valley, Blues for the Red Sun
Sleep - Dopesmoker / Jerusalem
Hawkwind - Space Ritual
Die Verbannten Kinder Evas - In Darkness Let Me Dwell
Lycia - Ionia
My Bloody Valentine - Loveless
Zoviet France - Shouting at the Ground, Shadow Thief of the Sun, Mohnomishe
Can - Tago Mago
Neu! - Neu! 75, Neu! (self-titled)
Faust - Faust, So Far, Tapes, IV
Captain Beefheart - Trout Mask Replica
King Crimson - anything from their debut (In the Court of the Crimson King) up through the 80s
Jimi Hendrix - Are Your Experienced?
Led Zeppelin - just about everything
Pink Floyd - Piper at the Gates of Dawn, Meddle, the Wall
Nurse With Wound - Soliloquy for Lilith, and others
Coil - almost everything
Spacemen 3 - The Perfect Perscription, Playing With Fire
Spiritualized - Lazer Guided Melodies
The Velvet Underground - self-titled first album, White Light / White Heat
The Residents - Duck Stab, 3rd Reich and Roll
Aphex Twin - Selected Ambient Works Volume 2
Autechre - Tri Repetae, LP5, Gantz Graf EP
Squarepusher - Hard Normal Daddy, Music Is Rotted One Note
Miles Davis - b*****s Brew
Windy and Carl - Drawing of Sound
Tenhi - Kauan
In The Woods - Heart of the Ages
Of the Wand and the Moon - Emptiness
Backworld - Holy Fire
Sol Invictus - In a Garden Green
Dargaard - In Nomine Aeternitatis
Dead Can Dance - The Serpent's Egg, Aion
Primus - Frizzle Fry
Mr. Bungle - self-titled
Melvins - Bullhead
Ok that's enough from me for now...