Oh this is easy. I rode Flight of Fear at PKI back in '98 or so when it had the OTSR's. That one ride was complete misery. I got a migrane, felt really sick and had to leave the park to take a nap back at the motel.
I have ridden the ride a few times since they got rid of the OTSR's. It's one of my favorite steel coasters now.
I once made the mistake of hopping on Cedar Point's Blue Streak while suffering from some abdominal pain. I love the ride, but when every jolt (there are a lot of them on that ride) causes moderate to severe pain in your stomach it can be hell.
First time I rode Hercules it was definitely my last. I love wood coasters, but it was just way too rough, especially the turn over the lake. What made it worse was that we waited nearly an hour to ride it. It was the first time I had to take Tylenol after a coaster.
Wood? Coaster at Playland in Vancouver BC after it had gotten good and warmed up and I was in the very back seat. I took a gut shot that almost made me throw up. Subsequent rides near the front of the train were far more pleasant and that is one heck of a fun coaster.
Steel? Manhattan Express at NY NY, in Las Vegas. Guy said only $10 for a re ride, I asked him quite seriously if he was gonna pay ME $10 to ride it again and if so, he'd better add a few more zeros on the end of that figure... :P
Hurricane @ The Pavillion. I tried to enjoy this coaster, and while I enjoy reputed headbangers such as Iron Wolf, Ninja (SFoG), or Shockwave, I couldn't handle the combination of G-Trains and what I assume is poor maintenance.
Well it was Screamin Eagle at SFSTL. The road wheels would leave the track and slam back down, not over hills but at random times whenever it felt like it. However, last season I got back on it and had a great ride...repairs???
As of now it's Chang, probably due to the very hot weather I rode it in. PKI Vortex and GL Villian tie for second.
Steel: Jack Rabbit at Celebration City. Worst coaster wiplash I've ever experienced. I felt like I was living my go-kart accident all over again.
As for wood, the only one that gets me almost every time is the Boss. I rode it a week or two ago in the very front at night, and it still was as brutal as ever. I think I've only ridden it once where it was semi-enjoyable and that was during Fright Fest a couple of years ago.