User blog comment:Skardae/Key Cards/@comment-25600444-20151007004109

It should go without saying that Attack Nulls and "Pay 1 gauge, Draw 2 Cards" are basially the key cards to every deck out there but here are some of the RRs and above I think are Key Cards (Note: These are my ideas at the least so..... yeah);

Dragon:


 * Steel Fist, Dragoknuckle

Magic:


 * Magical Goodbye

HDs (Harder to define due the huge variation of possible builds BUT...):


 * Stranger Dilemma (It's pretty nice in anything)
 * There are no Places for You to Run! (Thunder Mine Decks)

Dungeon:


 * Brave Equipment, Glory Seeker

Ancient:


 * Divine Dragon Creation

Hero:


 * Hyper Energy
 * Rescue Dragon Mach Braver
 * Call, Super Machine! (Brave Machines)

Hope this helps.