As much as I love onions, I might say... Peppers are a bit more versatile. You have an ingredient that can add a small amount of sweetness or crunch to a dish or make it extremely spicy, with each type of pepper having its own different taste. You have dry versions of it like Schezwan which can make your mouth feeling a little numb, or white and black pepper each adding their own flavor to a dish.
You can stew, stuff, bake, dry, fry, grill and anything in between. It makes a very delicious Arrabbiata sauce, Salsa, and it's the main ingredient in all hot sauces.
It's also an ingredient in way too many dishes. From Omelettes, to Fajitas, to Stews and even some baked goods.