Forgive me... at first I thought you were being morbid.
Splash Mountain, I save it until last when possible because I can't pass up the ride, but I'm always afraid I'll get too wet and be uncomfortable walking around.
Sometimes it's the biggest and best ride in the park. Sometimes it's the one that's farthest from the entrance, so we can experience a lot of the park on the walk out. Sometimes it's a ride with no wait so we can squeeze in another one (El Rio Del Tiempo, anyone?)
We always end our trips at Epcot and the last ride is Spaceship Earth. If we ended at MK I would say Space Mountain. MGM, TOT Animal Kingdom, Dinosouar.
Man, I hate that last ride 'cause I know it's the last. So hard to enjoy it. Nevertheless, here are the rides I prefer to end my stay with, depending on the park:
We always leave from MK on that last day of vacation because it is open the latest, so the ferry across Seven Seas Lagoon is traditionally our last ride. We stand on the upper back deck, listen to the Disney music, and watch Cinderella's castle and Space Mtn. grow smaller and smaller. Must go get tissue now!