Older/completed series:
- Alexandre Dumas - The Three Musketeers, Twenty Years After, The Vicomte of Bragelonne: Ten Years Later
- J K Rowling - Harry Potter
- Jonathan Stroud - The Bartimaeus Sequence
- Elizabeth Peters - The Amelia Peabody series (I'm going to count this as completed, I think)
Current/ongoing series:
- Derek Landy - Skulduggery Pleasant
- Rick Riordan - Percy Jackson (Percy Jackson and the Olympians & The Heroes of Olympus)
- Kalayna Price - the Alex Craft series
- Ben Aaronovitch - the Rivers of London / Peter Grant series
There are several other current series I'm following, even up to and including pre-ordering each book as it becomes available, but I would hesitate to include them among favourites in the "really enjoy and have re-read / may end up re-reading" sense.
Dumas' The D'Artagnan Romances will always have a special place in my heart, being the books that defined my childhood and which I re-read multiple times while still in kindergarten, even though it's now a couple of years since my last re-read, while the Harry Potter books and, more recently, the Skulduggery Pleasant series are responsible for my utter immersion into fandom, gaining a number of good friends through fandom (several of whom have become "real life" friends), and are, basically, far more than just "books to read and enjoy and then move on to something else".