I always felt a Bugs Life was unappreciated. I mean, it has a lot of terribly annoying flaws (the main character is CONSTANTLY accused of being wrong/bad, while the audience knows he isn't, the supporting cast is weak) but the movie always rings true with me because, of all the Disney villains we've had, few have ever been as BRUTALLY PSYCHOTIC as Hopper.
Seriously, he has this whole "Apocalypse Now" vibe to him, and he's just got this brutal, viscious, violent streak in him that makes him really scary to watch. I think there's a scene where one of his men disagrees with him, so he crushes him to death under a pile of food. On top of that, he's actually unbeatable in the average sense--the main characters are incapable of defeating him, and it literally takes an act of God (in this case, God=a bird) to actually kill him.
Plus, he's one of the few villains who openly kills people. He's up there with Sid in sheer satisfaction. And on the topic of Sid...
Sid was the SHIT. I mean, we all know a kid who was like that, to some degree. Some of us even abused our toys ourselves, maybe just accidently, but still. Here we have toys given human emotions, personalities, and--evidently--the capacity to feel pain, and all of a sudden Sid is a sort of brutalistic Nazi character who sadistically delights in torturing living, breathing characters in a variety of sick and twisted ways. He's like Dr. Mengele of the Cartoon Villain world. To this date, no Pixar villain has ever really reached that level of pure, dripping sadism.