My brother, ToaKylerak, was the one who was into BIONICLE in the early years. I was like, 4 years old when I got Huki, and that's because we where in Europe and they still sold the baggies in 2003. ToaKylerak got Jala and Tahu, in which he both lost. Anyways, the first set I remember him having was Gahlok, followed by some of the Bohrok Va and Gahlok Kal.
I think I got into BIONICLE during 2004, when I was five. I got Toa Onewa and a red Vahki. In 2005, I got some of the Visorak and Hordika. I think Keetongu, which my brother got, was pretty cool.
I'll talk about 2006 some other time, because that was my golden and favorite day of BIONICLE.