I was just going through all my old books and found I had nearly every single one of these (I think I lack the last four), as well as nearly every spin off he wrote. I'm sort of tempted to do a "Goosebumps Challenge" now or something, where I read through a book a week or maybe a book every 2-3 days. They aren't that long, and I'm a pretty fast reader, I'd just need the time to do so.
I absolutely loved this series, and I think it's what fueled my reading when I was a kid. R. L. Stine once said that one of his goals with this series was to get kids into reading, and I can say this series most certainly did it. I really give a tip of the hat to him for it. I have really fond memories of walking into Waldenbooks (before it was bought out by Borders) at my local mall and heading right to the back of the store, where there were just shelves of the series. I became an avid collector, starting from book one and going from there, and really surprised myself at all the memories I had when I was digging them out yesterday. They are now arranged in order on one of my bookshelves.
Favorites? There were so many classics for me, but I'd say that the first 16 or so really stick out at me. "One Day at Horrorland" was great, as was "The Haunted Mask", but I have to say I think my favorite was probably "The Beast from the East". Really good blend of humor and suspense, and the thought of playing a deadly game of tag with giant furry blue creatures that can talk your language (and are rabid fanboys when it comes to the rules) is the most memorable to me.
