The two worst I've ever seen was a Clydesdale Mule, poor thing only looked nice from far away, his head was 1 1/2 times longer than his neck at least. He had almost no neck, and no back, and no hind quarters, so he was shoulders and head, with a tail on one end.
The second was Percheron crossed with a ASB (I think) the poor things back dropped when he was only 4 years old. Takes beginners for rides all the time, but as soon as the saddle comes off, you notice that there is something like a 10 to 12 dip in the middle. Funny enough his has lots of action in the front, and sticks his tail out when he walks "fast". But is very laid back, he was named Gunther.