Hmm as the above posters mention, it depends on what you mean by rare I could give a list of all my limited skins if you mean those by saying rare(if someone wants the full list tell me and I will edit the post) but I will post which ones of my collection I consider the most rare :).
It would be PAX TF, Human Ryze, Silver Kayle, medieval twich, grey warwick, all the 2010 world cup skins , silent sona and mundo mundo (I rarely see that skin anymore ;/)