I know this isn't very original, but Harry Potter is the only book series that I find myself able to read again and again. Every time I pick it up I feel like I'm a kid again and I remember how excited I was when the books came out each year.
I'm actually holding off on my fourth read through until my oldest daughter is old enough to really start enjoying it (which is I hope soon). I'm doing this so that I can be surprised all over again by the little things I forgot or missed.
I bought the first book for my oldest niece but she was not excited at all to receive it as a gift. I'm hoping that she will read it though once other people in her school discover it! I imagine it would be fun to read it with your own kids though.