Channel and Sneak Attack don't have similar rules text, for example, but they overlap in their ability to do broken things with big colorless creatures, so you might pick up a Sneak Attack to support your early-pick Channel, and then prioritize the legendary Eldrazi creatures, Blightsteel Colossus, and Sundering Titan over a card like Karn Liberated, which only works with the Channel plan
Based on the latest market prices tracked on TCG Stacked, Ringwraiths is worth about $0.77 USD for a regular copy and $3.36 USD for a foil copy
Supporters Three cards I have to justify: Olympia, Hooligans Jim and Cas, Lass's Special Three cards that need a bit more love: Buck's Training, Rival, Sage's Training Three cards that need to be in every Cube: Lysandre's Trump Card, Colress, Lysandre To be fair, there aren't really a lot of poor Supporters in the Cube
I may not be trying to build a cEDH deck, but this is a case where fast mana makes sense
Out are cards that help opponents or don't have ideal synergy or add to the major mechanics: Anvil of Bogardan, Horn of Greed, Dictate of Kruphix, Howling Mine, Temporal Mastery, Time Warp, Smoke, Sign in Blood, Mystic Retrieval, Dismal Backwater, Doom Blade, Damnable Pact