I'm more of a fan of 2006. I mean, sure, the sets were not as good as 2007, but at least it led to the Inika build, which (if used right) could lead to tons of possibilities when it comes to posing. Sure, the Inika are generic now, but back then, they (and the Piraka) were a big deal. And not to mention it brought some pretty complex titan sets in the years afterwords.
I also like 2006 story-wise. It really went back to the mythical feel that 2004-2005 went away from. It went back to an island, but this time, it had huge titans, enslaved Matoran who follow snake-like humanoid creatures called the Piraka, and we have energized Toa. Not the mention the Toa didn't fight animals or hordes anymore; They had a team of villains, NOT made by Makuta, who were literal gangsters.
I love 2006, story-wise. The sets were a bit gimmicky, but they brought important aspects of the BIONICLE building experience for me and many other MOCs. Not to mention 2007 showed the true potential of the Inika build.