If you're truly afraid, you might consider Gadget's Go Coaster in Toontown. That one is the tamest in the park. If you go that route, I would go right when Toontown opens (hour after the rest of the park).
Of the two (MB or BTMR), definitely Big Thunder. It's smooth, well-themed, and the 'easiest' of the adult roller coasters. If you do not like Big Thunder, DO NOT do any of the others.
Space can be intimidating because you can't see where you are going, BUT it is very smooth and there aren't really any big drops. There are three lifts right in a row, and the ride is basically tiny drop, turn, tiny drop, turn, tiny drop, turn until it's over. There is one moderately sized drop maybe halfway-three quarters of the way into the ride.
Matterhorn doesn't have any big drops, and has a top speed of roughly 19 mph, but because of how you are seated it feels much faster than it is. Because the ride is so old, the transitions are very abrupt and the ride is bumpier and 'rougher' in feeling. Thus you might say it is the most "aggressive" of the DL roller coasters. Note that there are two sides of the Matterhorn, and I find the Fantasyland side (the right side) to be somewhat less intense.
So between Space and Matterhorn, which is more intense will probably depend what you find to be more intimidating: riding in the dark, or riding in the sunlight on a ride that is ultimately more aggressive.