Space Mountain simply because I can ride it many times and other than it being super rough, I feel fine whereas after a ride on the Hulk, motion sickness kicks in and I feel awful and need to sit down somewhere quiet for an hour or two afterwards
The Hulk is a great ride but it is not close to the best rollercoaster I have been on (I am jaded since I am 45 minutes from Six Flags Great Adventure with Kingda Ka). Space Mountain in the other had is an experience to say the least.