I would have to say, The Canyon (I-30) in downtown Dallas. It allows people to get to Mesquite from either Dallas or Ft Worth. It's a 4-lane that has S-curves that pass through downtown Dallas. There are also HOV lanes that open and close, which is bad enough, but you also got thousands of people getting on/off of I-35 and I-45. You can imagine the traffic that runs through downtown Dallas to get to 35 or 45. It's a complete mess.
I never knew why it's called The Canyon. Maybe because of the sharp curves, and the hills.