My favorite series is Little House on the Prairie. I still think that's an excellent series for children to read because it is fun and it always teaches a lot about the time period Laura was growing up in.
For younger children, I love the George and Martha books by James Marshall. (I actually like anything by James Marshall. His books are so funny.)
The Frog and Toad Collection Box Set (I Can Read Book 2) are wonderful, too. I so wish there were more than four.
What are some other series that would be good for young children or older children?