Harry Potter by J K Rowling
Skulduggery Pleasant by Derek Landy
The Bartimaeus Sequence by Jonathan Stroud
The Amelia Peabody series by Elizabeth Peters
The d'Artagnan Romances (The Three Musketeers, Twenty Years After and Ten Years Later) by Alexandre Dumas - my absolute childhood favourite
Lots of others I've enjoyed, but the abovementioned are the ones I've either re-read (some more than once) or can easily see myself re-reading in the future.