An old pickup truck full of junk, a la Sanford & Son. These trucks are almost always driven by old men, and they always seem to drive 10 m.p.h. below the speed limit.
A Volvo;)
How bout a truck hauling a load of chickens to the processing plant? They’re all in those little cages, so your car is bombarded by iddy biddy chicken feathers which of course stick to the gobs of straight-out-of-the-chicken chickenshit! Time to head for the carwash.
An ancient, slow-moving, black-smoke-belching farm truck, loaded with a herd of pigs, pig shit dripping out the back, being driven by a drunken Panamanian pig-farmer on a winding, pot-holed dirt road through the mountains with no place to pass, has to be a major contender.
a crop duster.
The Batmobile. Those flames shooting out the back would be hell on the paint job.
Logging trucks.
I know a couple people who have lost family members when those cables holding the logs have snapped. If I’m ever in the situation where I’m behind, or even near, one of those trucks, I do the best I can to get away.
I did wake up once (as a passenger!) to see a semi heading right for our vehicle. Once my heart resumed beating again, I noticed it was being towed by another truck. 
We have trucks here that haul compost materials, and phew-eee!! They make garbage trucks smell like perfume atomizers.
Mostly because garbage trucks are enclosed, and the compost trucks have an open bed ( albeit with high sides ), and just covered with a tarp. On a hot day the smell makes you want to pull over and hurl.
What romanperson said.
Worst vehicle I’ve ever been behind? Back in the late 80’s I had a 1974 VW Super Beetle. I was driving I-75 South heading home from Atlanta. I got behind an early 70’s station wagon full from the back window to the back of the driver’s seat and from side to side with clothes. Oh yeah, the damned thing was driving on THREE spare doughnuts and one balding tire!!! It took every bit of the 53 horsepower I could muster to get around this idiot.
Being an ex truck driver,and yes I drove gravel trucks too or what I call dumpwagons.I always hated being behind little blue hairs in big cars crawling down a hill when I needed that momentum to get up the next big hill.
I haven’t seen anyone mention a snow plow yet. Around here, we have snow plow/gravel trucks. They drive along pushing snow, and drop down gravel at slippery spots.
Following one of those on the highway during a storm is a chore. Sometimes, they can’t even be seen until I am within 100 feet doing about 40-50 mph. The only way to know if I am too close to the damn thing is if I can see it. Then I need to slow down Plus, it is downright dangerous to pass them, not to mention illegal.
I know from personal experience that somehow ending up on the Turnpike totally surrounded by a group of police cars chasing a suspect’s car is not a lot of fun.
Interestingly enough, it’s not as bad as it sounds…