Rhapidophyllum hystrix MSI
This is the most cold hardy of all palms surviving temps as low as 0F, and probably 5 degrees lower. Even lower temps may cut this palm to the ground, but as it is a suckering palm, it may come back from freezes as low as -10F. The length of the freeze is important, though, and any prolonged freeze that low will freeze the soil may kill this palm. But leaf damage happens at or below 0F. This palm is unique in that it also tolerates a tropical climate as well, making it the most adaptable palm species in existence. Some argue Nanorrhops, because of its tremendous drought tolerance, is equally adaptable, but that's like comparing apples to oranges.