Always have garlic powder and 3-6 heads of garlic in my kitchen and I am not above roasting a couple of them heads at a moments notice so I can toss roasted garlic into anything.
I like every month or two buying a pound of onions, run em over the mandolin and spend an afternoon doing a nice slow caramelize on em. Reduce em down to a cup or so of dark brown amazingness, toss it in the fridge and add into a bunch of food just like I do roasted garlic.
Favorite in terms of "food with X in it," or favorite in terms of adding it? Because if it's the first one, it turns out I love me some damn onions something fierce. If it's just "which ingredient do you enjoy adding," eggs every time. So satisfying.
There are so many things I could say, but if I'm being totally honest, white rice. I could eat it plain, with butter, with so many different sauces, under other food, mixed into other food, at any meal, as a snack, even multiple times a day. I never get tired of it.
Any kind of pork, beef, or lamb fat. Using it as a replacement for butter or oils to cook or prepare ingredients brings a whole new depth of flavor to a dish!