I LOVED both Babysitters Club and Babysitters Little Sister (the Karen books). I don't have my Karen books anymore, but I still have my BSC collection - it's in my classroom at school, and the kids don't read them. I guess they are too dated.
They are re-releasing them with updated covers, though, so maybe that will help. They're VERY 80s, but still good stories!
I also loved Goosebumps and later the Fear Street series. I still have all of those too - the Goosebumps are in my classroom, and the kids do still love those.
And then, of course, Lurlene McDaniel was my favorite author from 4th grade until... really, my junior year of high school, when J.K. Rowling took her place LOL. That's the only YA/kid series that is still prominently displayed on my bookshelves at home, with many of the covers taped or in tatters because of how many times I read those titles.
I also loved the Ramona books by Beverly Cleary and many of Roald Dahl's books. It's fun being an elementary school teacher because I get to reread my favorites to my class each year. I always read Ramona the Pest, The Witches, Tales of a 4th Grade Nothing, and Mrs. Piggle-Wiggle (another good series). When we get back from break, we're starting our mystery unit, during which I get to read The Westing Game with my high readers - also fun!