Doom Metal

Depends on what kind of Doom you like/looking for...

For starters I'd get Type O Negative's October Rust for gothic Doom.
Other Doom bands I like are:
Officium Triste
Novembers Doom
Rapture
Old Anathema (Judgement and Alternative 4 are good but not a bit different)
Swallow The Sun
 
Some pretty obvious bands in there...but you didn't specify much in your opening post.

Revelation
Orodruin
Warning
Solstice
The Gates of Slumber
Hour of 13
Pagan Altar
Solitude Aeturnus
Candlemass
Trouble
 
Candlemass
Solitude Aeturnus
Warning
Solstice
Doomsword

Morgion
Evoken
old Anathema (Serenades, Pentecost III, Silent Enigma, Eternity)
My Dying Bride
Unholy
Skepticism
Esoteric
Dolorian
Mourning Beloveth
Ataraxie
dISEMBOWELMENT
Winter
Thergothon
Dusk
Novembers Doom
 
Mar De Grises, Daylight Dies, Katatonia, Saturnus and Mourning Beloveth are the only doom bands I listen to much. The heavily Sabbath influenced bands and funeral doom has never really appealed to me. The Angelic Process are cool too, doomy/shoegaze kinda band.
 
I love doom, but few bands impress me. Here's what I listen to:

Katatonia
Novembers Doom
Lacrimas Profundere (only "Memorandum" and "Burning:A Wish")
Anathema
My Dying Bride
While Heaven Wept
Mourning Beloveth
Swallow The Sun
Long Winter's Stare (definitel an acquired taste. I know few who actually like them. I love em.)

Definitely try Lacrimas Profundere.