On the topic of Doom-related mcs, MF DOOM (and his umpteen million aliases) is a personal favourite of mine.

See:
- Danger Doom - The Mouse and the Mask (MF DOOM and Danger Mouse)
- Madvillain - Madvillainy (MF DOOM and Madlib)
- MF DOOM - Operation: Doomsday (MF DOOM circa 1999)
- Viktor Vaughn - Vaudeville Villain (MF DOOM alias circa 2003)