I probably have to rewatch the Toy Story movies, but i seem to be in the minority that vastly prefered the 2nd film to the other two, i don't know, it simply spoke more to me, and the hype was so large for the 3rd film that i was actualy expecting to like it better than i did.
Either way, that one and the Lord of the Rings trilogy are the ones where the quality is the most consistent, however i still have to watch some of the most popular trilogies like Imagination, Before, Once Upon a Time and Colors. Here's what i can rate so far:
1. The Lord of the Rings
2. Toy Story
3. The Godfather (I didn't find the 3rd one bad enough to drag the other 2 down)
4. Man with No Name
5. Original Star Wars Trilogy
Man with no name's the only one that truly improved with each instalment, the first one was good, but then the Trilogy turned into greatness midway For a Few Dollars More (or 20 minutes in).