I heard the black chrome Darth Vader minifig is kinda special? Got that from ToysRus.
Also have the chrome TC-14 minifig.
If Toa Hagah are considered rare - I have Iruini 2 1/2 times and Norik 4 times.
Then there's my complete '01 Kanohi collection.
My rarest complete Bionicle set probably is Lesovikk, though, considering that it uses parts only included in that set and as far as I know it wasn't sold in my country.