It is close beetween the Star Wars Jedi Apprentice books and the Harry Potter books! But in the end, I would decide the Star Wars books. You can't beat a little acyion with Obi-Wan Kenobi and Qui-Gon Jinn
Except that all ended thanks to Episode I
But let's not get into that. What are your favorites? Please post and let us know why you like those books!