Although Grapes comes close, I'm not sure any card in TexP is truly a staple. While I could see it becoming closer and closer to being a staple as more people use demons, I don't think 3 Nails is a staple either simply because the threat of it makes people wary of playing all demons and consequently they aren't dominant. Kind of similar to how banding defenses never became dominant in the days when AoCP was the only way to beat them--the threat of AoC kept people from using banding defense exclusively.
Although there's many cards that find their way into most of my decks, the only ones I can safely say have been in every serious deck are Son of God, New Jerusalem, Angel of the Lord and Christian Martyr;, beyond those I believe every slot in the deck to be open.
