Obviously, a black deck should run some spot removal. But the question is, which is the best? The 3 I can think of originally (with pros and cons are) (in the order I'm thinking atm): Shriekmaw
Pros: 3/2 evasive body, can be easily recurred, blocks regen, only 2CMC for pure "destroy" effect, hits Emrakul.
Cons: Doesn't hit black or artifacts (large number of creatures). Chill to the Bone
Pros: Hits almost every creature that will see play in EDH.
Cons: 4 CMC (others are 3) Eyeblight's Ending
Pros: Elves are a "weenie" creature type, and do not see as much EDH play.
Cons: Doesn't hit a number of generals, including Mayael and Nath. Rend Flesh
Pros: Hits a lot of creatures.
Cons: A fair amount of Spirits see play in EDH.
Obviously, a black deck should run some spot removal.
Really? Neither of mine do (well, not of the traditional instant/sorcery variety, at any rate), and they seem to go fine.
That said, both Eyeblight's Ending and Rend Flesh are popular choices, from what I can see. Maw doesn't seem worth it unless you can recur it. My personal favourite, although not one you see that often, is Ashes to Ashes. It may be sorcery-speed, but it laughs at recursion.
Ashes to Ashes - EXILES 2 (nonartifact) critters for 3 mana and 5 life; a bargain in EDH, and stops recursion. Amazing.
EDIT: Viperesque already mentioned it, but I'll keep it here, because it's amazing.
Sudden Death - -4/-4 is enough to kill most things, but it's the split second that makes it invaluable.
Slaughter Pact - my monoblack taps out a lot, so the ability to get removal out of nowhere really helps.
Altar of Shadows - expensive (14 mana to kill your first creature), but in a control deck, it's repeatable instant-speed creature removal that eventually helps you ramp.
Drana, Kalastria Bloodchief - a creature, so somewhat fragile; while the effect is expensive, it is repeatable, instant-speed, and lets you make a whole bunch of mana into a whole bunch of hurt to your opponent.
Don't forget that Drain Life effects can be used as removal for creatures as well as finishers for players, and still gain life (to offset all the life costs you've no doubt been paying). Good luck!
As a connoisseur of fun, interesting matches, I still to this day have not been able to craft that "perfect deck"; the deck that I can play and have fun over time, doesn't get boring, but simultaneously is fun to play against. I honestly don't think it exists. It's like a unicorn. A ninja unicorn.
A great spot removal card in my Maga EDH aside from Sudden Death.
Phthisis for the pump generals that get out of hand (Kresh, Omnath, etc). Sudden Death for small utility generals that get annoying quick (any dual color "When you cast a (color) spell, ..."-type general, Azami, etc)
Otherwise, as mentioned earlier, use Drain Life effects like Consume Spirit, Corrupt, Tendrils of Corruption, Profane Command, etc. They double as spot removal and late game win conditions, while providing that additional boost in the jams to stall until you get more removal or win cons in hand.