When your primary win condition is attacking with Vengevine, having a two-mana artifact that mills your opponent while doing triple duty as a graveyard enabler and sacrifice outlet was outrageous
Honorable Mention #1 (#12) - Electric Revelation I know that it's not the same power level, but the first thing that Electric Revelation reminded me was of another flashback card powerhouse named Faithless Looting
The card itself is actually pretty interesting, as its the first-ever time that the creature types Cleric and Wizard were directly mentioned in the rules text of a card, and has actually since been updated be a Human Cleric Wizard instead of just Cleric as printed
Special variants like serialized cards, old Reserved List prints, and limited promotional cards can be even rarer due to extremely low print runs.
Perhaps the motivation to create the subtype is strictly flavor-driven, with unclear plans on how this could be activated via rules in the future