Chiming in with a reminder about some MG classics - never forget the classics! -
*anything by Beverly Cleary, specifically anything starring the Quimby sisters
*anything by Barthe Declements, specifically the "Nothing's Fair in Fifth Grade" series
*Anne of Green Gables - for those who really love the written word, and aren't afraid to read "deeply."
and last, a more "modern classic" - E.L. Konisberg's "From the Mixed Up Files of Mrs. Basil E. Frankwiler." Oh, and "Jennifer, Hecate, MacBeth, William McKinley, and Me, Elizabeth" - same author.
I'm loving the other suggestions here, too!