2001/2002 for the classic Bionicle aesthetic. Would say pretty much all sets from this period were fucking golden. Especially the rahi from 2001 are legendary. They had this really charismatic clunkyness over them, also using technic construction elements for decoration, having figures designed around a technic feature, that sort of thing. This was also when Bionicle was still a sub-theme of lego Technic.
Toa Nuva from 2003 were also pretty great in themselves, and I loved them back them, but now I think they were kind of a step in the wrong direction desu. After the Nuva nothing really worthwhile came out until 2006 in my opinion, only series that all had the same uniform body-build and silver weapons. Piraka were sort of nice, but especially Axonn and Brutaka you really need to get. Both beautifully designed.
2007 was also exceptionally good though, and really brought the old creative feeling back into it with the weird animalistic style of the Barakki and robotic scubadivers for Toa.
Personally have a weak spot for some of the 2008 sets as well, but not all. Vamprah especially I thought looked great, never took him apart since I got him eleven years ago. Also the vehicle sets were really nice, and brought back some of the technical complexity that was lost over the years.
2009 had some nice sets, but overall not really great in my opinion. They had a tendency to look better on the box art than in hand.
2010 made everyone cry. Don’t ask why, you don’t want to go there.