My toaster oven. By far it is the one small appliance that sees use nearly everyday for something.
Sometimes I'm reheating pizza, toasting a bagel, using it as a small oven when I don't feel like waiting for my big oven to preheat. It's so versatile I don't know if I could live without it.
You should check out air fryers then. They're basically toaster ovens with a fan for blowing the hot air around. Amazing for making things nice and crispy!