2001: To this day it's still my favorite year, and I freely admit I'm biased due to nostalgia. The worldbuilding and tone relayed by MNOG were mindblowing to my six year old self and the experience still inspires me to this day. I became a fan too late to get any Tohunga sets but I loved their charm and character quirks. The Toa were fun, especially mah boi Pohatu, but even better were the Rahi and their functions--it always disappointed me that the big Technic contraptions of old never really returned in subsequent years. I'm also a huge fan of how colorful and varied the sets were, something later years ditched almost immediately much to my chagrin. How amazing would it have been if we continued to get purple, yellow and teal sets for the remainder of Bionicle’s run?! I have a permanent display of all the 2001 sets in my room and it brings me tremendous pride and happiness to see it every day.
2002: Didn't read the comics but I really enjoyed the Bohrok Online Animations that filled in the gap. The Bohrok were fun for their day and I have memories of setting up Toa like bowling pins and mowing them down with a curled up Bohrok. Didn't care much for the Toa Nuva thanks to the hideous masks. Cahdok and Gahdok were fun as the last hurrah of the Rahi sets.
2003: I loved the Bohrok-Kal for some reason, so much so that I ended up buying five of them. They were cool, but even cooler were the Titan sets for that year. Pewku and Makuta were both a delight--though I wasn't a fan of the increasingly dull color choices for some of the sets, Gukko and the Rahkshi in particular. Didn't like the Matoran redesign or the movie which butchered Takua and Jaller's personalities. I prefer the Bohrok-Kal half of the year more for that reason alone.
2004: On an objective level, putting all nostalgia aside, this is my favorite year, but that's almost entirely due to story rather than sets. As much as I missed the vibrant colors of Mata Nui, the Toa and their setting were well-developed enough that it didn't matter. The Vahki weren't that great and the Titans were a bit hit or miss (actually all miss if I'm being honest, save for Lhikan and Kikanalo). Legends of Metru Nui remains my favorite movie for the incredible soundtrack and the fact it managed not to ruin any characters.
2005: This was the year where I started to lose interest in Bionicle, but I didn't notice it at the time because I was still quite engaged with the Toa Metru's story through the books. I didn't buy very many of them but I liked the Hordika and Visorak quite a bit, and Keetongu was pretty good too. The third movie was unremarkable, but not bad per se. Don't have much else to say about it.
2006: The shift in setting to Voya Nui was jarring, and at this point I was growing out of Bionicle and no longer really cared about the characters or the quote unquote "mature" storytelling. The Piraka were insufferable and obnoxious even to my ten year old self, a far cry from the terrifying primal evil of 2001's Makuta, and I was getting sick of the deluge of villains who were reputed to be able to murder ten Toa at once in gory ways yet somehow always managed to lose to that year's headlining Toa team. The mysticism I had loved about the older years was completely gone. When I saw the Toa Inika, those blobby faced, gangly abominations of the Matoran I loved so dearly, I lost interest entirely. I was also, to some degree, frustrated by the lack of answers to the series' mysteries, something that wouldn't be remedied until much later. To this day I’ve never bothered to read up on the story from this year and indeed the rest of the Ignition trilogy other than what I’ve read in the comics and BS01, so my Bionicle knowledge has something of a gap starting here.
2007: I still had a comics subscription from Lego Magazine, but I was pretty indifferent to what was happening. In retrospect, the sets are actually pretty cool and I wish I'd picked a few of them up after watching Eljay's recap reviews. The Mahri had respectable designs and so did the Barraki, and I’d love to pick up Maxilos and Karzahni someday. Otherwise I don’t have much to say about it.
2008: Still had that comics subscription but I think I only read each issue once and then forget about them. Didn’t care for any of the sets or the story which as far as I could tell was just year three of mindless battle after battle with the only difference being the environmental setpieces.
2009: This is where things get interesting. I randomly decide to browse a Bionicle wiki to see what stuff I’ve missed and read about Mata Nui’s awakening as a giant robot. WHAT. HOLY CRUD WHAT HAVE I MISSED. I play catchup and start following the story again just in time to buy the Mata Nui Glatorian set and catch The Legend Reborn, which I actually enjoyed quite a bit back in the day, though I’ve yet to rewatch it to see if it still holds up (it probably doesn’t). Also--I know the Toa Mata Nui set is often maligned in the community, but I still want that thing to this day. Badly.
2010: It was a decent conclusion to the series and I’m just grateful it happened at all. I went into the Stars line with no expectations and just bought them for nostalgia’s sake, nothing more, so I don’t have any strong opinions on them save for the ball joint cracking issue. The Mata Nui Saga was enjoyable too.
2015: A return to form in looks but not in spirit. On a surface level all the ingredients were there: six amnesiac elemental heroes crash land on a tropical island, forced to unite to face down the evil Makuta. But the similarities end there as far as I’m concerned. I could complain about the utter childishness and vapidity of this relaunch all day but I don’t want this section to dominate my list so I’ll cut it off here. For what it’s worth I enjoyed the Toa sets (save for the runt of the litter. RIP in peace Pohatu), but didn’t care for the Spooky Scary Skull heads. I might’ve picked up the Protectors if they had any individuality to them, but alas.
2016: I don’t feel the need to comment on this at all. I only paid attention due to morbid curiosity, and I shed no tears over its untimely demise.