Does anyone have a "signature" card (like your favourite)? Mine is the Red Eyes Black Dragon as it is a key in my dragon deck.
In my Twilight Angels deck: Darklord Asmodeus. I love pulling off combos with Covering Fire, Dark Hole and A Hero Emerges.
In my Volcanic/CyberDragon synergy deck: Blaze Fenix, the Burning Bombardment Bird. Formed some excellent combos with Future Fusion and Volcanic Counter that saved me more times than I can count.
In basically every other deck: Sacred Phoenix of Nephthys.
But for my 'true ace', I'd definitely say Sacred Phoenix of Nephthys.