Middle Grade Recommendations?

What is your favorite middle grade novel?

I think my next project might be a middle grade novel. Or maybe a YA without the "mature" content. Susan is always saying, "Write what scares you." Well, right now, what scares me is writing a novel where my kids come away thinking it's okay to um...experiment. Then again...maybe when they're older and I'm exeriencing teenaged drama every day.

So, back to the question. I need to do some more middle grade research. What do you recommend?