Oh man this question just hit me with a wave nostalgia.
The first manga series I ever bought was Dragonaball, don't remember the issue it was but it basically covered from when when future Trunks defeated Frieza, I was probably 10 years old.
At the time manga in the school was very limited, and we only had dragonball, everyone had to be on a borrowing list. I remember the teacher had to scribble out any panel that showed Kid Goku's penis lol. I was very shy, timid kid and always hated asking my mom for stuff so one Christmas eve I took the bus by myself to the local mall and with all the money money I saved up I bought that one manga. It was an awesome experience! After that I never really had the need to buy manga since I discovered I could see it online for free.
One day I really want to buy all of the Berserk series, Air Gear, Blade of the Immortal, Naruto, Dragonball and Gintama just because I loved all those series so much. Nothing beats reading manga/books in paper format in my opinion.