Well thats pretty straight forward, and to be fair this would be a lot easier to list my favorite book series to Urban Fantasy, Techno-Thriller, Sci-Fi, MilSciFi, etc.
But thats not what the Meme says so I guess I'll have to boil it down.
If I had to pick a favorite current series I'd go with the Dresden Files, by Jim Butcher. They are, simply put, crack in literary form. I read a lot of series these days, but Butcher's Dresden Files have held my attention for almost a decade. *Thats* staying power.
If you're interested and looking, start with Storm Front