Bionicels duh =P. But the old City sets were very nice, not to mention cheaper, MM sets, I kinda got a crush on PM really late, Aquaraiders, Alpha Team, Dino Attack, Johnny Thunder, even Agents, Blacktron. You name it, if it's old it's great. I pretty much like every theme fro unique pieces. Storyline totally influences whether or not I am a huge fan of the theme.