Penguins

Penguins cannot fly due to their tiny wings and huge bodies. Penguins are heavier than the heaviest flying birds. Penguins do not have hollow bones unlike most flying birds which adds to their weight. Most flying birds have hollow bones to be lightweight. Also, flying birds have curved wings while penguins have straight wings(like flippers). Curved wings keep the flying birds aloft and keeps air under it's wings while straight  wings are more suitable underwater where they can be used as propellers. If ever a penguin attempted to fly, it's wings would need to flap 35 times every second. A penguin cannot generate that much energy at once.

Flying is not suitable for penguins because they eat fish that live deep down in the depths of the ocean. Flying would not be an advantage to help them fish.