This a community build, to let everyone get together and talk about which cards are a MUST in ALL or to MOST EDH decks.
I am going to throw in a couple cards for all colors, and lands as well, and provide a mini "WHY is this card good," description. and if you have any cards that you fit should be in this list, provide a comment of this card and "WHY this card is so good in EDH."
COLORS:
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RED:
--Bogardan Hellkite = Hellkite gets flashed into play, rips apart board positions, sometimes sending damage straight to the face, and then might even get a swing in!
-- Kiki-Jiki, Mirror Breaker = Kiki-Jiki clones every turn and hastily too! He makes every creature we have literally twice as efficient.
-- Shattering Spree = can be used for mass artifact removel, I mean who doesnt like that!
-- Wildfire = has a nice symmetric effect, angering all of your opponents equally. It's good for getting rid of most creatures, and a few problematic lands as well.
-- Fork, Wild Ricochet and Reiterate, and Reverberate = Great copy effects if you want to get your opponents good!
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BLACK:
-- Demonic Tutor, Vampiric Tutor, Diabolic Tutor, and Beseech the Queen = Should be in every EDH deck that has black, for tutoring
--Cruel Tutor, Demonic Collusion, Grim Tutor = are also good tutors too!
-- Necropotence = great card draw engine!, but be careful, it could put a target on you, because everyone will be scared of all the cards you will be drawing!
-- Phyrexian Area = another great drawing engine!
-- Mutilate and Damnation = are certainly fantastic at destroying entire fields full of creatures.
-- Shriekmaw and Hero's Demise = great cheap removal
-- Liliana Vess and Sorin Markov = are both absolutely absurd sources of powerful reusable card advantage.
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GREEN:
--Sakura-Tribe Elder = Great cheap mana ramp creature!
-- Civic Wayfinder and Yavimaya Elder = alos great cheap mana ramp guys!
-- Eternal Witness = possibly my favorite green edh creature, or creature in general. brings back any card from your graveyard to your hand! thats FUCKING AWESOME!
-- Primal Command, Tooth and Nail, Worldly Tutor, & Sylvan Tutor = green tutor cards are always fun!
-- Life from the Loam = great recursion card!
-- Krosan Grip = greatest green artifact/enchantment removal
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WHITE:
-- Yosei, the Morning Star = Sure go ahead and kill this guy, he benefits from dying :P
-- Akroma, Angel of Wrath = She just straight up OWNS!
-- Wrath of God, Day of Judgment, Rout, Austere Command, Armageddon = These are just some very great Board Sweeps
-- Swords to Plowshares, Condemn, Path to Exile = Fucking Sweet removal cards!
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BLUE:
-- Counterspell, Force of Will, Spell Crumple, Flusterstorm = just a few of the great counter spells
-- Phyrexian Metamorph and Clone = Being able to do Whatever your opponent does always helps.
-- Ponder, Brainstorm = Helps your top deck, and card advantage!