We've had this discussion at home many times, most especially after the new movie comes out. It's getting harder and harder to be able to rank them. We, too, have always put Bug's Life last, but even at that bottom ranking it is still far better than a lot of other animated flicks. I think Bug's Life just doesn't hook your emotions like some of the others do, but it's still magical and fun.
Here's my ranking, and mine will likely change often, too.
1. Toy Story - I agree with Gone2Disneyland, it's pure magic. It also touched a nerve with me because as a kid I thought my toys came to life when I wasn't home.
2. Toy Story 2 - Yes, more action and the like, a great film, but the original is still my favorite. I love old reliable Woody.
3. Monster's Inc - I love how they took the original childhood fear of a monster under the bed and put a twist on it.
The next few I will put in a rank order because of the theme of the thread, but in all truthfullness they rank so close together that they could change places and still be accutate.
4. Wall-E - The character of Wall-E is just so human like, and I felt a lot of emotion between he and Eve. Brilliant.
5. Finding Nemo - there's just so much warm sentiment in this movie, and strong family ties. And the ocean was animated so well.
6. Incredibles - again, strong family theme, and we like superhero movies in our house. And Frozone was just so cool!
7. Cars - I was pleasantly surprised. I expected not to like this because it was about Cars, but I love it.
8. Ratatouille - I loved the movie, but I felt a bit more emotion with some that are ranked higher.
The least favorite is Bug's Life, as I just don't feel the emotional connection with it as I do with some of the others. But it's still a great movie, and a 1000 times better than anything Disney has put out in their animation department in the last decade or more.
My favorite Pixar short has always been Geri's Game. That is pure brilliance, you almost believe it's two different people sitting there.