I'm going to give my obvious answer and say Kings Dominion.
As well as being concrete heaven with large open spaces providing little shade, it has my bottom 3 woodie coasters in Rebel Yell, Hurler and Grizzly; three of the worst pieces of crap I've ever been on. I never got to ride Shockwave, but I could imagine it being awful as well. The rest of the rides are mediocre at best (before i305 was built).
Add in rude staff (the ride operators complained about us to our faces when we tried to use some fastpass things we were given to use), generally crap personal experience with other people in the group that day (not the parks fault, granted, but it all affects my memories of the day), and a dilusional KD manageent team ("We ARE better than Disney World"), and you have my worst overall park!
My secondary choice goes to Sea World in Australia.
Sea World looks like a park that was abandoned about 20 years ago but thy never told their employees. The layout is awful, the animal enclosures are nonexistent, and the shows are also seemingly nonexistant (no signs and nothing to tell you if anything is actually going on that day). The 3 rides that make up the park are all pretty crap, and the food is by far the worst I've ever had in any park, ever (the chipswere still cold and raw in the middle, and the burger was like a piece of cold leather).
Honorary mentions go to Happy Valley Shenzhen and Camelot.