The elusive Inside Tour Trans Blue 2015 Kaukau, as per my avatar. For those curious it's mask #167
This one came from the fan media days that Lego hosted a while back. I was invited to go, but unfortunately due to the timing being so last minute, I didn't end up going- the person who went on our behalf was kind enough to send me the bionicle goods that were handed out.
I've also got about 24 of the trans neon green skull Scorpio masks and some of the gold Kulta mask polybags (also came from the above situation) which round out my rarest mask collection I suppose.