There's really only two candidates for me, the two cards I've built casual decks around ever since Ice Age was printed and continue to do so - Enduring Renewal and Zur's Wierding. It's hard to judge which to go with, from a casual point of view Renewal has been tainted by it's use in combo decks but Zur's Wierding is a little less fun because it's about stopping people using their spells. Therefore, regardless of the combo connotations I announce:
Enduring Renewal
My Butterfly deck (Enduring Renewal/Giant Caterpillar) was a staple deck I had constructed for about 5 years, just mixing cards in and out as I thought of them.