Bought a NIB Muaka and Kane-ra on ebay for $75. The price would have felt a little steep if they were used, but I'm a sucker for the feel of opening boxes and bags and building with brand new parts.
If I have to choose only 1 item, the platinum Avohkii seems like an obvious choice to me. That being said, being able to say you own every single Kanohi mask would be quite the feat.
I've always had a soft spot for the Exo-Toa, but Takutanuva takes a close second as he is made of another good titan set, and the reasoning behind the combiner model didn't seem quite as forced as some in later years.