One of the most over-looked rides in DCA is Heimlich's Chew Chew train. As you go through the ride different scents are released. Heimlich's narration is funny for parents and children. All of Bug's Land is well themed!

In DLR, I would say Peter Pan's Flight! When I went to the parks when I was younger I bypasses this ride for the big thrill rides. As an adult, I look back and say.."What was I thinking?" You could spend an hour describing all of the awesome details on this ride. For me, I feel like I am a kid again every time I ride it.
As for overrated? I would say all of Toontown. While I understand the theme of a cartoon land, after going through the town once there is no reason to return. My DD5 always wants to bypass it when we are in the parks, which makes us happy!

They could do wonderful things with that area of the park.
Also, Monsters Inc....ugh..talk about being underwhelmed!