Generally speaking, I'm not a huge fan of middle-grade books. While I can still enjoy a YA book, even though it's not as complex or deep as an adult novel, middle grade is just too young for my tastes. There's the odd book that I enjoy, but for the most part, the stories and characters are just too shallow for me. And I don't mean that as a disparagement towards those who do love it, just that it's hard for me to stay interested in a lot of the middle-grade books I've picked up.
That being said, there are a few that I do love. Most of them are from my own childhood, but some are ones I've discovered as an adult. And since this isn't a genre, but an age group, I've broken the books down by genre.
Fantasy & Sci-Fi