My first sets were Nuju Hordika, Nokama Hordika, and Rahaga Iruini. I only really got into the story when I got Hakann and Hewkii Inika. I later went back and purchased (read: my parents purchased) Toa Lhikan & Kikanalo, Keetongu, and the special edition Toa Hagah Iruini & Norik combo.
Many years later (as in, this year), I bought Tahnok-Kal, making that my "oldest" set in terms of date produced, and my newest set in terms of date obtained. My newest sets in terms of date produced are Toa Ekimu and Umarak the Destroyer; I didn't get any of the Beasts.