Hmmm... Hard question with so many ridden coasters. One of the few coasters that come up in my mind is the Georgia Cyclone in 2011. I don't know what they did, but the ride was so rough that I was totally surprised on it's roughness. Definitely one of the rougher woodies I've ridden to date.
Another one that's worth mentioning is the Coaster Express in Parque Warner. Extremely rough track, very dull layout and Intamin trains. You are bouncing the entire ride up and down due the bumps in tracks.
Steel coasters is going to be a totally different story. With some visits to Japan and China. I have ridden some extremely terrible rides. The worst definitely has to be the Screamin' Squirrel coaster that's located in China. With it's terrible maintenance and adjustment on the brakes/trims, the trains were flying over those saxophones. Creating an insane ride but also extremely rough: (Best seen on the offride starting on 1:30)
These S&S rides aren't the smoothest rides due those transitions but without proper trimming it seriously goes over the top.
Another coaster that was beyond terrible was the Crazy Roller Coaster in Quanlin Resort. The cars were extremely uncomfortable and the ride made it even worse. To be honest, I can't really recall any extremely terrible or rough steel coasters from the US. Okay, many of the SLC's are in terrible shape. But I haven't ridden anything that was in such shape that I would never want to ride again.
Actually, now when I think it of it. I have ridden a coaster that was so weird and bad, we kept on re-riding it on checking how bad it actually was. We just couldn't believe it was THAT bad.
I'm talking about Nitro in Dennly's parc, A coaster by Preston & Barbieri. The ride has weird shaping combined with weird suspension on the cars, which ends up in some even weirder turns and possible painful moments if you don't brace yourself.