My vote for this one was rather simple; it's the card I never leave home without:
MIRACLE
If there's a Light card in the game that DOES shed light in the darkness, it's this card. Going from the edge of defeat to suddenly winning the game is a feeling that never gets old.
Not to mention this + Heal = insta-EM.
It is beyond any doubt that this card is worth spending all your Light quanta on. There's also the part where I couldn't stress enough how many times this card saved me from losing matches; whether I got out-rushed, never got the card(s) I needed/wanted, or just did something completely silly.
This card is always in my Rainbow deck, and I'd be an absolute fool to take it out.
Honorable mentions go to Archangel and Blessing; because those, too, are awesome cards. 8)