I'll try not to have Harry Potter for every other entry in this challenge, but I may have it several times.
I have thought of what serieses I've read, and there were quite a few. In fact, I'm reading one now. I loved all of them. This of course makes sense, since if you don't at least like the first book in a series, you obviously won't read on. That has happened.
The reason why Harry Potter is the favourite, is because for years, I just wanted to live in the world. When I first started reading this, there had been four books out. Some of you may have read the whole series one after the other, but let me tell you, waiting for those three books was exhausting. I still remember buying every book. At first, I just bought the first one. I was at the local small foreign languages book store (had closed by now), and I was looking for something to read. Not a huge collection. So I wanted to pick up Lord of the Rings, but then I spotted the Harry Potter books. A friend of mine recommended them to me. She had an 8-year-old son, and they were practically tearing the book out of each others' hands. I though that if a 39-year-old woman likes it, a 20-year-old can too. On my way home I had a long bus ride. I first looked at Lord of the Rings, but decided I wasn't in the mood for something so serious. So I decided to begin Harry Potter. On the first page Rowling described how Petunia's neck was so long, so that she could spy over the fence on the neighbours. I laughed out loud, and was hooked. The next day I bought the rest of the series, and gobbled the whole thing up in a week.
Then it was a looooong wait for the next book. I read a lot of fan fiction meanwhile. First Harry/Hermione, then Drarry, Snarry, Harry/Lucius. I bought every other book on the day of release. I joined Pottermore in Beta. Slytherin, BTW. Of course. I joined Facebook groups. I wrote fan fiction. I wrote with a green pen at the university. Obsession didn't even cover it. I think I'm getting off of it now, but still, forever a favourite. Once I have a kid, I'm going to get them on the Harry Potter bandwagon.