The pantry/fridge-cleaning exercise shows which ingredients we keep for vanity (I love Japanese packaging,) to feel exotic (who else found rose-scented water and saffron?) or because it seemed like a good idea at the time (two-for-one deals on something you rarely use.) It also sheds a lot of light on which ingredients we keep in constant rotation.
Even if you're not participating in the Cure, you are probably well-aware of what ingredients you use most. It's hard to answer the question of what one ingredient is essential in any kitchen. I have a handful that are neck-in-neck: olive oil, butter, chicken stock, onions, black pepper and sea salt. Those things alone don't make a meal, but they help make whatever is in season taste delicious.
Leave us your list of essential ingredients, we might just surprise each other.