I would have to say, with what I've read, the Mindbender at Galaxyland.  After the second loop the back car came off the track and started fishtailing which caused a few people to be flung from the back car.  It came to the third loop and couldn't make it, due to it losing a lot of speed.  It started going back through the previous track until it finally ran into a concrete support struckture, killing three and nearly killing a fourth. (all this info was gathered from wikipedia http://en.wikipedia.org/wiki/Mindbender_%28Galaxyland%29#Crash) 
  
Another notable one that was caused due to the rider and not the park was at Waterworld USA(I'm not sure if we are counting water parks here so if not sorry, but i still consider this horrific  ).  A graduating high school class was trying to beat the record for most students on a waterslide at one time.  They all ignored the life guard and rushed onto the slide filling it to near quadruple recommended capacity.  The water slide then collapsed killing one girl and injuring 30 more.  I found out about this from a newstory a few years back while watching youtube. heres a link to what happened http://articles.cnn.com/1997-06-03/us/9706_03_water.slide_1_water-slide-waterworld-usa-banzai-pipeline?_s=PM:US