I really don't like rides with drops, but I have tried all at WDW except for Tower of Terror and Expedition Everest. I haven't done EE because we haven't been there yet since it's been opened, but will be trying it next trip. Tower of Terror I just know I'll hate.
Rockin' Roller Coast is my favorite because you don't have those drops and don't get that horrible feeling in your stomach!! My next choice would be Thunder Mountain Railroad, Space Mountain, and then Splash Mountain.
As for Mission Space, I've done it twice and will never do the full thing again. Next time it'll be on the tame side. I do not get motion sick, but after doing Mission Space, I was feeling sick for 2 hours after that. It is an awesome ride and I'm glad I did it, but not again.