For a start:
Rejuvenation (http://elementscommunity.org/forum/index.php/topic,34369.0.html)
Quantum Locket (http://elementscommunity.org/forum/index.php/topic,25665.html)
Please provide a brief description of your reasoning. Update coming shortly!
I have 6 criteria for a card to be Great.
Balanceable, Creative, Thematic (including mechanically thematic), Standalone, varied, and has a niche.
To showcase a card it must be Great and improve the metagame in a significant manner.
Rejuvenation
is easily balanced by adjusting whether it can target creatures or by adjusting the casting cost.
is a creative take on healing
is thematic (regeneration is a growth based healing)
is mechanically thematic (regeneration is useful in multiples)
can be used as a standalone PC card
is varied with both offensive, defensive and counter defensive uses
has a niche of permanent lobotomy
Beyond that Rejuvenation helps balance the metagame by creating more alternatives to Deflagration when combating Discord. It was well placed in one of the elements that would benefit from an alternative to Deflagration.
Quanta Locket
is easily balanced by adjusting the casting cost and activation cost. Although it probably is balanced already.
is a creative means of quanta fixation
is thematic (Other uses generic names)
is mechanically thematic (quanta fixation should come at a cost that scales with the versatility used)
is meant to be used as a standalone quanta fixer
has varied usages depending on the scale (number of elements) being fixed. Mostly it enables quad and quint decks.
has a niche of quanta fixation
Quanta Locket opens the game to the possibility of Quad and Quint decks while at the same time creating much more flexibility in the quanta ratios for decks. 1:3:6:6 decks would be possible to design with Quanta Locket.