Only you can save mankind was a childhood favourite. It really resonated with me back then, haven't read them in a while though.
I always thought the quality diminished steadily with the next two though - Johnny and the dead seemed to trickle to a halt, and the story with Johnny and the bomb, although entertaining, again seemed really weak in comparison to other children's books he's written.

But as a set of young adult books, they're fantastic. They fit the target market perfectly!