Oh boy. This is a hard choice. Ever since I was young I've read the Redwall series. I might ahve to say those are my favourite series, though "His Dark Materials" by Philip Pullman is a wonderful trilogy. I love the Narnia books too and have read those countless times as well. Those are the oldest trilogy I love. My parents read them to us in the car on long trips.
Hm... I love the Harry Potter series, and ofcourse the Alias books. The Narnia books are good too. But I love the Deptford Mice Trilogy by Robin Jarvis. Oh, and I love the Twilight Series by Stephanie Meyer.