Well, I've hosted and cooked the last seven, and they've been awesome. That's not me being all arrogant. That's people coming back every year, and for Christmas, and for Easter. So either my cooking is super awesome, or no one else wants to host.
So I can't really think of the worst Thanksgiving meal--my mom and Grandmother always made stellar meals, so it wasn't one of those. I even ate the Brussels Sprouts as a child. I also loved spinach and liver, so take however much stock you would like to in that last statement.
I imagine that the worst TG dinner I've had would have been any I've eaten in camp. Because really, when any meat hangs out in a steam tray for hours, it's pretty gross and dry.