For me it was getting a simple Master Control grand taille; my first serious watch besides a 1970 Speedmaster I bought when I was 18. Was around 30 when I got the JLC and it marked not only a change towards a more mature style of watch but also established a broader interest in horology. Liked it so much I got a gold version of the same model just a year or so later. The blued second hands on both were stunning. Sadly, both watches have since been passed on: the gold in exchange for a Rolex 16570 polar, the steel because I acquired a 666 Ingenieur.