Honorable Mention: Visara the Dreadful
A 5/5 flier for 6 who isn't likely to attack due a fantastic ability. 3 black mana limits her to heavy swamps, but unless you are facing the most aggressive of decks she will dominate the board.
10. Living Death
The anti Day of Judgement. A re-animators dream. I've never actually played with this card, but I can see the power. Living End is close, albeit a weaker version. You can do things to sack your own creatures, or possibly mill yourself. The surprise element combined with the comeback factor is what makes this thing amazing.
9. Abyssal Persecutor
With this card you are a sacrifice away from the dumbest creature ever. I mean dumb in a great way of course. There isn't much that beats harder than a 6/6 flying trample that can't be tapped down a la Desecration Demon. It's the reverse Platinum Angel. Honestly there a ton of ways to kill this thing yourself, especially in black.
Kind of a build around card, but a flavor-fully wonderful staple. Destroys all weenie decks everywhere. Works well in multiplayer, with Stuffy Doll, Fungusaur you name it.
The finest defensive card you could want. For a single mana you get a flying re-generator. Typically regenerating is pricier than this. Doesn't stop trample, but it wasn't meant to. Aggressive cards don't want it. Those are literally the only drawbacks. It can't attack like Drudge Skeletons, but it's cheaper and stops all opposing creatures.
6. Go for the Throat
My personal favorite removal not named Swords to Plowshares. It was at a disadvantage in a heavy artifact block, but when playing most kinds of games(non Eldrazi, Affinity) it will be perfect. Hero's Downfall, Victim of the Night, Murder, Terror and Doom Blade all have their places. This is my choice.
5. Phyrexian Arena
Not as quick as Necropotence, but the fact is you still get your draw step. It's easier to splash, works better on turn 3 than Necro, cheaper than Dark Confidant and is quicker than Yawgmoth's Bargain, Bloodgift Demon. Just solid.
4. Phyrexian Obliterator
This card just kind of seems like cheating to me. How is it this card came to be? 5/5 for 4 usually has a drawback of some kind(Green not included). Restrictive mana cost? Fine! Want to block, you better have some 0/6 walls in play.
3. Mind Twist
Card advantage is key, and there isn't much better than the ole twist. The random part of it is what really makes it broken(It would still be broken if it wasn't random). Hymn to Tourach is probably number 2 in terms of the best discard card, but this here is the king. I love that they re-printed it in 4th edition and Revised thus insuring the price will be forever low.
2. Dark Ritual
Holds it's own with Lighting Bolt and Ancestral Recall as the color staple. Poor Healing Salve...Maybe if it was 13 life, it would be mentioned with Giant Growth. Ritual however is one B away from being a common Black Lotus.
1. Contract from Below
So you have to be playing for Ante. Otherwise you can't possibly argue drawing 7 cards for 1 mana isn't the best card in the game. If Inquest magazine can make Ancestral Recall their number one card of all time, then this most certainly is better. Wheel of Fortune, Griselbrand, Braingeyser are all so powerful, but this is just better. It does involve high stakes gambling...