Infinite energy generation via the candle or cup aside, I would probably go with the blanket. Passive temperature immunity would be OP, and also useful in daily life as well. Also, the "perfect size" stat means it can always be carried around in a pocket or something. The candle is a close second, but it lacks the blanket's portability. I wouldn't want to carry around a lit candle all the time, so it would only be useful at home.