It's one of my favorites from Pixar, though not my top favorite. It probably ranks second. I think they came up with such a good concept, and they created an entire WORLD off of this concept, which is pretty impressive. I was originally looking forward to the sequel, as I felt there were a ton of possibilities they could spin off from given the sheer scale they created with the first movie, but now that I'm hearing it's a prequel well... I'm not saying I'm disappointed, but it just seems to be the easier of the two ways out in terms of writing a story. A prequel you already have established characters, and established canon that will occur after it, so you're a little more confined in terms of what you can write. With a sequel at least, you can create almost anything new.
I just think the concept of Monsters having their own world separate from our own is awesome, and they were pretty detailed with the science of how many doors they'd need in the door vault to cover the planet. THAT in particular impressed me, as they mentioned they did all their math, and exactly how much space was taken up as well.
