First one that comes to mind is Paramaecium's Exhumed of the Earth. Oh man that's some heavy stuff! So let's talk about stuff like: Winter; Funeral; Thergothon; Esoteric; Norrt; diSEMBOWELMENT; Unholy ... etc. Heavy and slow. Slow and heavy. No Black Sabbath here ... way too fast.
Does YOB count as doom? Regardless of what genre they fall into, they are relatively slow and excellent (imho)
I won't pretend to be very highly versed in this field so maybe these are obvious choices, but two that immediately come to mind are... Catacombs-In The Depths of R'lyeh Ahab-The Divinity of Oceans
Wonder what you might think of Esoteric's utterly insane albums The Pernicious Enigma and Metamorphogenesis? These are two of the heaviest and CRAZIEST doom metal albums I've ever heard. Hard to find but totally worth it.
I have The Pernicious Enigma which is a classic of course! Epistomological Despondency, the debut, is worthy as well. I like everything I've heard from them.
The thing that first got me into this style of music was Eternal Frost and Into Darkness by Winter. This is the band that started it all ... forging their lonely path to practically zero fanfare when nobody else was doing this extreme style of plodding heaviness. I worship at the altar of Winter!
Wow, I thought I was the only guy on these boards to even know of Paramaecium! Exhumed of the Earth is a masterpiece, as is Thergothon's only record. I like to believe that the cold, frozen earth of Scandinavia swallowed the Thergothon guys after they finished their one album. They had perfected the genre in only one album, so they could go back to the permafrost whence they came. I have a friend who's a real metalhead, and he felt like he was physically suffocating when he heard the Thergothon demos (early version of their record). He thought he was in a coffin buried in freakin Denmark or something. He couldn't handle the doom.
It's too bad they lost Neyland as a vocalist... but I'll still check out what these guys come up with next
I would have to say that Paramaecium album is the heaviest music I've ever heard. There is really nothing else like it. And those dudes in Thergothon were teenagers! Seems like I read Thergothon only did two gigs and recorded that one album. They're also certainly in a class of one. I just love this stuff!
I would say that Skepticism has taken the low, slow funeral doom style to its extreme limits. The vocals are almost subsonic and the guitars are down-tuned so much I'm surprised the strings don't just fall off the nut. Nortt comes close, I think, but they (he) actually has a bit of that black metal edge sneaking in.
I guess we ought to mention St. Vitus and Trouble too as masters of doom. Vitus' Burial At Sea is one of my favorites. Really creates a great, "watery" atmosphere.