Most of DOOM's current fan base came into being when three of his albums leaked simultaneously:
* King Geedorah -
Take Me to Your Leader
* Madvillain -
Madvillainy
* Viktor Vaudeville -
Vaudeville Villain
(Incidentally, the Jaylib album also leaked around this time. Both the Jaylib and Madvillain leaks were unfinished versions of the album. You can check the leaks if you want just for the novelty of it, but you're really not missing anything if you just buy the retail albums).
The King Geedorah album showcases DOOM's production skills. The beats are great on there, but DOOM himself only raps on a few songs.
The first Viktor Vaughn album has no beats by DOOM, but he shows his ass lyrically.
Madvillain = Madlib + DOOM. It's a masterpiece on both the beats and rhymes ends, and definitely the best DOOM album that's commercially available right now.
Operation: Doomsday! is still DOOM's best album, though. Produced entirely by DOOM at the tail end of a five-year alcohol binge. It was released in 1999 and 2001, and is out of print because of a lawsuit over the cover art. Rhymesayers was supposed to reissue it, but they haven't gotten around to it. You can probably track it down online for some ridiculous price, or you could just download it. I don't see anything unethical about downloading something you can't buy from the artist.
I'm pretty sure
Mm..Food is still available - I see HipHopSite has it. It's a terrific album that got ruined by terrible sequencing - sitting through skits, the instrumental suite in the middle... eeyukh (c) Pusha T. There are still ten great songs on there, though.
There's also the Dangerdoom album and
Venomous Villain.
Venomous Villain is an EP with eight minutes of DOOM rapping, plus many more minutes of bullshit. The first few lines are "Dub it off your man / Don't spend that 10 bucks / I did it for the advance / The back end sucks", which is pretty much all that needs to be said. If you can stomach the beats and Iz-Real's wack-ass crew, though, there are some great rhymes on there. Chronic's a big defender of this EP, or at least he used to be.
The Dangerdoom album is more of the same, I guess. I don't like Danger Mouse when he does rap music. The album's still very enjoyable, though (Doom's actually the main attraction, he's not rapping on beats to help some other guy's crew get shine). Except for that song with Cee-lo and the Talib Kweli verse.
If you're into Doom's beats, check MF Grimm's
The Downfall of Ibliys. There's also the
Special Herbs series of instrumentals, though if you don't like listening to loops, check
Special Herbs and Spices (Grimm rhyming over some of the special herbs) and
Special Blends, which is exactly what it sounds like.
This would all be remiss if I didn't mention KMD, DOOM's old group. Check 3rd Bass's "The Gas Face", KMD's "Peachfuzz", and the entire
Black Bastards album.
Hell:
http://stonesthrow.com/madvillain/mfdoom-discography.html