I am a total wimp when it comes to rollercoasters, but I love all of the coasters at Disney World. My big fear is the first big hill (excessive fear of heights) and none of the coasters have that.
Big Thunder Mountain is actually pretty tame. The first hill turns around a corner, so doesn't seem too big. It never really gets going really fast. It does jerk you around a bit, so if you prefer a smoother ride, you might not like it.
Space Mountain is also pretty tame for a coaster. I think the darkness and jerkiness are what turn some people off. I like the dark because then I can't see how high I am!
The Rockin' Rollercoaster is my favorite. The corkscrews and loop are a blast and it doesn't have to rely on a huge first hill because of the unique propulsion system. It is extremely smooth, no jerkiness. The hardest part is the take-off. I get nervous every time I stand in that line watching the cars take off. It accelerates so fast that it looks unnatural! I've found the best way to deal with the g forces of the take off is to scream as loud as I can all the way down the tunnel. I think expelling all of that air helps to alleviate the wierd feeling you get in your chest when you are subject to those kind of g forces.
I've only been on Splash Mountain once and I hated it. Again, that is from my fear of heights, so if heights don't bother you, it should be fine. I'm thinking of going on it again this summer, though. I was so nervous about the last hill on Splash Mountain that I didn't stop to enjoy the rest of the ride. Now that I know when the drop is coming, I think I might enjoy it more.
Hope that helps!
MikeP