Scorpia by Anthony Horowitz
I used to love the adventure and excitement of the Alex Rider series (of which Scorpia was the fifth installment), the thrill of imagining a teenager like myself in such crazy situations, having to be so resourceful and brave. Now I appreciate more how Anthony Horowitz did show the mental trauma that Alex must have experienced – seriously, some of those situations must have messed him up! But when I was younger this was at the back of mind; instead I was enchanted by the thrill of his adventures, if you can call them that. My favourite of the series was Scorpia, despite the other two books featuring the organisation not being my favourites by quite a large margain. The last chapter, when you think Alex is safe and it becomes clear that he obviously is not, was so tense when I was reading it in my final year of primary school. A book, and a series, that I will always remember with fondness (tinged with tension, or course).
Close second: the Artemis Fowl series.