Force of Negation is a premier answer against Omni-Tell since it can counter Show and Tell for free on their turn, stopping Omniscience or a reanimated Atraxa, Grand Unifier before it hits the battlefield. Consign to Memory can counter colorless spells and triggered abilities, giving you another way to interact with their artifacts like Ancient Tomb-fueled plays in a pinch, though it will not stop Show and Tell or Omniscience themselves since those aren't colorless. Daze is weak here since they typically have ample mana from Ancient Tomb to pay the extra 1, so it rarely slows them down meaningfully. Prioritize Force of Will and Thoughtseize to strip Show and Tell or Omniscience from their hand before they can assemble the combo.