Probably a) Boomerang at WOF or b) Eurosat at Europa-Park.

Shockwave, Six Flags Great America, Gurnee, IL. I'm only putting it on the list because after you got off the ride it felt like you had whiplash (neck strain). The coaster was very jerky.

When I used to live in U.S, I made a list of my top 10 least favorites, but I will include some that I have ridden in Europe. This list has number 1 being the worst I've ridden and number 10 being the best of the worst that I have ridden.

1. Gouderix at Parc Asterix
   
2. Dragon at Adventureland
   
3. Bandit at Movie Park Germany
   
4. Wild Beast at Canada's Wonderland
   
5. Ninja at Six Flags St Louis
   
6. Flashback at Six FLags Over Georgia (No Longer Exists)
   
7. Anaconda at Walygator Parc
   
8. Grizzly at California's Great America/Kings Dominion
   
9. Mean Streak at Cedar Point
   
10. Son of Beast at Kings Island
    

I haven't been to an abundant amount of parks. Only 3, Busch Gardens, Dollywood, and Cedar Point. I'll do it this way, (since I can't decide over one or the other)
Favorite Busch Garden's ride: Griffon Cool Fact: I went on in 32 times in a row the first time I was able to go on it.
Least Favorite Busch Garden's ride: Verbolten, 3.0. Hurting factor, made me sort-of dizzy afterwards.
Favorite Dollywood Ride: Wild Eagle (I believe that's it's name). Cool Fact: I am the only one in my family who could/would ride it.
Least Favorite Dollywood Ride: Easily Tennessee Tornado, 2.0 Hurting factor, mucho headbanging, and almost lost my glasses going over a loop. xD
Favorite Cedar Point Ride: Millennium Force Cool Fact: The tallest rollercoaster I've been in. (It would have been Top Thrill Dragster if it hadn't been being greased and raining a few minutes later).
Least Favorite Cedar Point Ride: Mean Streak 1.0. Toughness 10, Cool factor, 7.56, and hurting ability, 403.9. (I will NEVER go on Mean Streak again!!!)

^ lol mean streak isnt even that bad... at least i wasnt when i went to cedar point, but that was over a year ago

^ Yeah, If you want rough, SFFT's original Rattler, THAT WAS AWFUL!!!

OzIris at Parc Asterix.

*hides behind concrete shield*

I just couldn't keep my eyes open. I couldn't enjoy it, through the six or seven times I rode it. I rode it so many times to try and get an enjoyable ride, but no. It felt like I blacked out two or three times. Every ride.

I'd give it a 3.5.

To me it was. When you have constant headaches that DON'T go away, that kind-of beating around will lessen the chance of me going on it again. xP

Pegasus at Mt. Olympus -1/10

Bump bump bump <

MY PERSONAL TOP 5:
Out of 89 different coasters I have been on, here's the 10 worst!
(yes I am aware I said top 5, I have dishonourable mentions too)
Dishonourable Mentions:
- Vekoma Boomerangs
Only been on a few and don't plan on riding any more, these are terrible and not worth waiting for.
- Rougarou, Cedar Point
This ride is really bad. Rough and very lame. At least I didn't have to wait for it.
-Raging Bull, Six Flags Great America
This is admittedly a smooth ride, but this layout is TERRIBLE. Allows no airtime whatsoever and has no good elements. Also not worth a 40 min. wait.
-Wicked Twister, Cedar Point
I loved V2 at Great America, so what the heck happened here? So darn rough! The headbanging is VERY gratuitous.
-Random Kiddie Coasters
I just ride these for the credit. They suck tho haha
Now....
5. Timber Wolf, Worlds of Fun
Bumpy, rough and uncomfortable. Not good. Sitting up front did minimize these problems.
4. Banshee, Kings Island
Kill me. I DON'T CARE YO. This ride was super rough, and a very unpleasant experience. The restraints are so uncomfortable and the headbanging is insane.
3.  Screamin' Eagle, Six Flags St. Louis
So rough I cried from the pain. OUCH. Felt this one all week.
2. Ninja, Six Flags St. Louis
Well, I had to wait 3 hours because of my party...and then got put in the very back row. Rough as can be, rattling and headbanging and an AWFUL layout. Yuck.
1. DEMON, Six Flags Great America
Not nostalgic, just nasty hard to ride. The layout is admittedly fine, but it's hard to CARE when you are being injured on this thing! This gave me bruises, abrasions, and migraines, and I really don't want to talk about it.
This is my opinion so feel free to disagree!