Demilich In Modern, were currently seeing a resurgence of UR Delver-style decks brought about by Modern Horizons II all-stars Ragavan, Nimble Pilferer, Dragons Rage Channeler, and Murktide Regent
Why was Toolcraft Exemplar pushed so hard
Meet Fog, the namesake card for todays category of all-damage-preventing spells
The best cards are ones that either make other cards provide mana of any color like Chromatic Lantern, tutor for lands, or provide multiple mana in any combination of colors
Cards that prevent creatures from untapping, as these are so disliked by the majority of players that resolving one actually gets more combat damage dealt your way, not less
We just don't get viable 2-cost mana rocks very often, with the last universally playable one being Arcane Signet released 5+ years ago now