There are many misconceptions and it seems like the answer to this question varies based on personal beliefs. The truth is that there are so many really rare LOL skins that are quite hard to find at the moment. Some of them were launched during different events while others were offered as rewards for specific achievements accomplished by the gamers. However, just as with every single skin for any game out there, it is a guarantee that one skin stands out as the rarest one. The skin that is the rarest in League Of Legends at the moment is the Pax Twisted Fate skin.
This LOL skin was released during a special Pax based event that was held some time ago. It is really hard to find it on the internet, especially for free since there are not may that are offering it. There have been eBay auctions in which the skin for the Pax hero was sold for over $1,000. This makes it the most expensive of all skins. It is even more expensive than Championship Riven, which is valued at around 250 EURO at the moment. With this in mind, make sure that you are attentive where you download the skin from. Make sure that you receive what you pay for or what is promised.