I'm going to rate the years using a 1-10 system, 10 being the best.
2001 - 10/10 - 2001 was a perfect year, it served as a great introduction to the Bionicle, the mctoran, turaga, toa and rahi are all fun sets, mnog is a pure masterpiece and the collectible masks are great, I'm still trying to collect them all
2002 - 7/10 - A good year, the bohrok were cool + iconic and the krana were good evil counterparts to kanohi, the toa nuva were ok, Tahu and Kopaka stood out, while the overs looked odd, the nuva masks, except for the hau and akaku weren't that good to me, the titans were average, but atleast Boxor did make black mctoran arms cheaper on Bricklink!
2003 - 7/10 - a mixed year, the bohrok kal were cool, krana kal gave awesome new colours for the krana, and who coudl forget WMKK, SSKK, GPKK and VMKK, Iconic pieces right there! the Rahkshi, Makuta and Takanuva were excellent sets, but the mask of light movie was severley disappoiting, Jaller went from brave war commander to dim wit 5 year old and Takua went from Brave adventurer to plain weirdo, the rebuilt matoran are atrocities too, both in story and out.
2004 - 8/10 - Metru Nui was great, the toa metru are awesome innovative sets, it's a shame there weren't collectible masks, (imagine a metru mahiki in red!) but kanoka were fun collectibles too, the novels became more mainstream allowing for the story to be conveyed better, and the titans were good.
2005 - 6/10 - While I don't think it is as horrible as others have pointed out, 2005 could have just not existed and not much would be different, but the visorak and horidka were ok sets, the combo models made from the titans were exceptional, while the titans were rather bland, plus 2 different gold kraahkan variants came from this year.
2006 - 7/10 - villains and titans were amazing in 2006, Axonn and Brutaka are amongst my favourite sets of all time, the piraka were neat, but the Inika and the voyatoran were horrific, the Inika masks were just disturbing and irrelevant and the voyatoran didn't even change the mask colours!! (just imagine the red mahiki from earlier!)
2007 - 7/10 - A good year, Toa, villains, matoran and titans were all good, nothing much to say or complain about, Matoro's death was pretty sad.
2008 - 4/10 - what were they thinking? I think this was more irrelevant than 2005, the nuva had their legacy ruined with those disturbing sets and the matoran have the blandest build known to man, the makuta were good, but why did they have to ruin Makuta himself by having him as some species, he was unique and cool, in my headcanon I see them as upgraded rahkshi.
2009 - 6/10 - Great potential, ruined ;(
2010 - 1//10 - the only good thing to come out of this was a few new pieces.
2015 - 8/10 - A good start, the sets were good, the story had room for improvement, but so did the first year of other themes, so an overall good year
2016 - 3/10 - A sad fate for bionicle, Journey to one had amazing animation but the writing is just horrific, the uniters I find bland, Ekimu and Umarak were cool sets, and the summer wave never even reached me so I cannot judge them