I wouldn't go down the Darkglass route- Their stuff sounds incredible, but sounds more modern to my ear than what stoner/doom requires. I have a Wren & Cuff Elephant Skin, which is basically their Tall Font Russian with an overdrive built in to send it into crazy territory. It's pretty beastly (it's Troy Sanders' signature pedal, you know it's going to be heavy), has to be one of my favourite fuzzes. I've also gotten a Fuzzhugger Doom Bloom recently- it has a very cutting germanium fuzz tone, but no clean blend. The controls are such that you can still keep a lot of low-end (the control scheme isn't like any other pedal I've used), but I feel it needs an OD in front of it.
I do recommend an Earthquaker Devices Acapulco Gold too, even if you get something else as well. It's only got one control (level) and it's based on part of a Sunn Model T head, so you know it's built for doom. Although you can't tweak it, it's quite sensitive to how you're playing and whatever is in front of it in the chain, so you can get a few different sounds out of it- The EQD guys recommend having an EQ in front of it to change its behaviour. Also, it's pretty cheap because it's such a simple pedal.
(I swear to god, I'm not an EQD rep. They're just pretty awesome).
The Dwarfcraft Necromancer is pretty nice too, especially if you want to be different and get something not muff-based. It's a superfuzz style fuzz with a built in 3 band EQ.
Pretty damn doomy if you ask me.
