There are very few limited skins in the game, most are legacy so that means they're usually back once a year for a quick cash grab.
The old Christmas and Halloween skins are no longer limited and are available annually.
Limited skins include:
- Human Ryze (preorder retail version)
- Black Alistair (preorder digital version)
- Silver Kayle (retail package)
- Championship Riven (last limited skin)
- King Rammus (given to closed beta testers)
- Urf the Manatee
- Rusty Blitzcrank (it sucked so much they removed it forever)
- Riot Singed (this one they randomly made limited unlike the others because reasons)
- UFO Corki
- PAX line (rumour has it that they have a contract with PAX to not sell these)
- Victorious line (Jarvan, Janna, Elise, Morgana, the rewards each season for being Gold+)
Then there's Judgement Kayle which was given out to anyone who made their accounts in Season 1 or earlier.
And that's it, those are the only "rare" skins because almost anything else will be available at least once a year and the ones which aren't (Traditional skins) can still be acquired through mystery gifts.