I wish I could say 2007 with the Mahri plot, but I can't because all connector sockets from that period seem to keep snapping, which is a major detriment to me (poor Hahli, Jaller and Matoro can never be completely rebuilt again...not that I would) So for me it's the year I got truly invested in the series - 2003. Yes, the Bohrok Kal were clones but as a kid whose first Bionicle was a Tahnok (and who LOVED Bohrok) I didn't mind. I saw them as "squad leaders" and those little mini-comics made them seem more worrying. Though admittedly at the time I couldn't grasp the story due to a lack of means to learn it all. The Mask of Light arc was fun, too, and the shadow titan Makuta (and Takutanuva) were really fun to have, if a bit antiquated by modern Bionicle building standards. That said, I enjoyed 2005 a lot too with the VIsorak, Toa Hordika and titan sets. Roodaka and Keetongu were particular favourites for the period, and Sidorak only fell short because I'd wished his head would have been made with a mask like it looks in the movies. Seriously, why did they never make any movie-style masks? I'd kill Miserix to get a proper movie Kraahkan or Kaukau Nuva.