My favorite Middle Grade series is Peter and the Starcatchers by Dave Barry and Ridley Pearson.
My favorite middle grade series was Gary Paulsen
Josephine Against the Sea is my favorite right now.
My favorite Middle Grade book would be the first book of Land of Stories.
Percy Jackson is always a classic
Old school: The Chronicle’s of Narnia, The Book of Three
New school: Percy Jackson, Harry Potter
BOB by Wendy Mass, Rebecca Stead, and Nicholas Gannon :))
The Giver series is my most favorite Middle Grade series. It’s really what started my love for reading.
Eva Evergreen -semi magical witch by Julie Abe
Oh man, as a sixth grade teacher they are so many great options! I read through The Jumbies by Tracey Baptiste earlier this year and it’s an amazing ride. A beautiful blend of fantasy with just the right amount of creepiness for my students!
Harry Potter is still the best!
I actually help judge a statewide middle-grade books competition, so I read a LOT in this genre. My single favorite series is probably The Land of Stories series by Chris Colfer.
I love the Artemis Fowl series 
The 39 Clues Series!
the land of stories by chris colfer!!
My favorite middle grade series would definitely be “the Babysitters Club.” I read all of the books when I was young, and I’m excited to pass on my collection to the next generation.
Oh my, there are so many Middle Grade books I love! I think right now the Incorrigible Children of Ashton Place series is my favorite. I love the word play.
Coincidentally, one of my favorite Middle Grade series is Artemis Fowl by Eoin Colfer. I’m excited to see what Myles and Beckett grt up to in The Fowl Twins!
City of Ghosts Series by V.E. Schwab! 
My favorite middle grade book is Ella Enchanted by Gail Carson Levine.