This topic makes for such an interesting list,and finally had to register just to comment on it. I would think that the list would center around a few particularly painful coasters, but it seems that worst coaster is just as personal and subjective as choosing the best.
I always assumed I disliked Arrow loopers more than the average enthusiast. I have an extremely long torso, which makes the crap Arrow restraints either stick out comically far or compress my spine beyond what should be physically possible (often starts with the former, ends with the latter). Based on this thread, it may in fact be the exact opposite. Since many other over-the-shoulder restraints cause me pain (Intamin's current hard restraints are currently the worst offenders; they leave nasty marks after the ride), I may be more tolerant of Arrow's often painful rides than many.
My choice for worst coaster would have to be Shockwave at KD. That stands out as notably awful. I'm also going to have to give an honorable mention to Katun at Mirabilandia, for being the most disappointing. I may have just ridden at a bad time, but it served as an example of how rough a B&M can get if not given proper TLC.