...is it normal that I think a poll like this is actually kinda detrimental to understanding what defines Bionicle? It makes it look like there should only be one worthwile thing that forms the entire line. G1 is practically a Rorschach test for the fanbase - everyone takes one look at it and values it differently, but that doesn't mean what the majority sees is the real core. It's like looking at a human body and asking: "Which one organ keeps us alive? Is it the stomach? The lungs? The heart?"
Bionicle is definitely a combination of everything listed - maybe everything to different degrees, sure, but finding out these proportions through a poll doesn't mathematically make sense anyway (if 95% of people like cake because of its sweetness, then cakes must be made out of 95% sugar, right?). Each element has its own functions that are important, and it's difficult to discuss them in any order because of how they all tie together; pick one over the others, and you miss the big picture:
a) Sets: What Lego actually needs to succeed - and is how most people got into Bionicle im the first place (if not everyone). Release bad sets, and the line fails instantly - but it isn't necessarily what everyone loves about it. On the other hand, I've known ex-fans who only care about the sets and nothing else. The toys are definitely the core from a business perspective, but not directly what made it successful or too impactful. When you favor this over everything else, you get G2; great toys, but placed in a context where there is no real reason to care about anything.
b) Story/Plot: Bionicle's story isn't the most well-written thing on earth, but it's definitely capable of resonating with people. This the core from a very broad perspective, as it serves as the vehicle for all the other points to shine: sets need to be sold somehow, characters need to do something, and the worldbuilding needs to be used in some form.
c) Characters: Though many don't realize it, some of the best tales have characters driving the story, and not the other way round. Most of the time Bionicle's characters weren't the most three-dimensional, but they served their purpose. Characters serve an important yet overlooked purpose to everything: sets need to sell something, a story always needs to be about someone, the universe has to be explored by someone, and the tone needs to be expressed through people, not events.
d) Worldbuilding/Atmosphere: I think this point is kinda cheating, because it's easily two different elements placed together for being what clearly sticks with everyone the most. It's what Bionicle excelled at during its early years. Worldbuilding creates atmosphere, but they aren't the same. Worldbuilding is the universe your characters inhabit; a universe without someone to marvel at it is duller than you might think. Atmosphere is what the story, characters and universe make the protagonists and audience feel. These are the core of Bionicle for the impression it gives, burning it into our minds. But these poins can only be justified by all the others - otherwise, it's a hollow experience.
If you haven't voted yet, please distinguish what you like and what you think is most important. If you agree with what I said, there isn't really one choice.
Edit: I just realized how bitter my wall of text reads, moments after posting it. I'm not, before anyone gets on my case, but I do think it hinders discussion to be forced to pick one option and stand by the opinion that that's what matters most in a multi-faceted franchise.
TL;DR: Pick your favorite brach off a tree, but don't call it its "core."