Airborne weather radar without a doubt. Before radar arrived inadvertent penetrations of thunderstorms were the cause of accidents several decades ago. From this came the term "Jet Upset". Just imagine an airliner flying blind at 35,000 ft in areas where huge storms are known to reside and the radar is inoperative. The chances of running into a thunderstorm in some parts of the world including the Pacific Ocean is increased significantly.
Hit one of those monsters and the danger of loss of control is there.