Biggles, Jennings, Molesworth and Swallows & Amazons are all ones I remember fondly. I also enjoyed things like Alan Garners Weirdstone of Brisingham, Moon of Gomrath etc, Susan Coopers Dark is rising series. Richard Adams Watership Down, Plague Dogs and Shardik. Plenty of others I can't remember now and the suggestions for some of the scifi are pretty good as well, Asimov is very accessible for a younger age as is Larry Niven.
More recently my kids have been through the Harry Potters, the Alex Rider series, Skullduggery Pleasant, Percy Jackson, Charlie Higsons, truly scary, horror ones and various others.
Thinking on it I remember liking the Hardy Boys and Alfred Hitchocks Three Investigators series as a kid as well.