That is a tough call. I did It's Tough to be a Bug ONE time, I never feel the need to experience that again. I've never bothered with Stitch, read WAY too many bad stories about it here on the DIS.
However, I'm going to say my least favorite is Splash Mountain. Now before you jump on me . .. . . it is a FANTASTIC ride, wonderful scenery, great job of telling the story, but that drop scares the bejeezus outta me, AND my DS12 LOVES the ride. DW, and DD21 are more scared of it than even I am, SOOOOOOO .. . . . . you know who gets to ride with DS12 every time right! At first, I (mistakenly) thought that after going on it 15 or 20 times, I would become desensitized to it. Yeah, right lol.