I haven't read everyone's responses, but I am 10000000000% positive that I am the only person who is going to say Tomorrowland Transit Association (previously the Wedway Peoplemover).
I just love the breeze it produces in August and there's never a long wait!
I have to say Space Mountain. To me, there is no greater feeling then when the ride starts and you enter into the big room with all the stars. That is one of the reasons I love Disney!!!
Test Track!!!!!!, Crushin Gusher at Tyhpoon Lagoon was great too! I don't know if the wave pool at TL is a "ride" but we LOVED that, too. It's Tough to be a Bug was good, too!
Mission Space. My son and I rode it for the first time this year (actually, twice but not in a row). But, I have a certain fondness for Spaceship Earth. Just something about that ride through the Epcot globe...