I don't know if it is the most hated (yet), but wild parsnip has become an issue for at least one local course, and it is spreading like crazy around the area. It has become really bad along some of the highways where mowing frequency has been cut or eliminated.
I'm surprised no one else mentioned this. It mainly shows up during times of drought but this stuff is NASTY. From my understanding when this stuff gets on your skin and then has exposure to UV, it turns into some type of acid.
"Once the furocoumarins are absorbed by the skin, they are energized by uv light on both sunny and cloudy days. They then bind to DNA and cell membranes, destroying cells and skin. Parsnip burns usually occur in streaks and elongated spots, reflecting where a damaged leaf or stem moved across the skin before exposure to sunlight."
http://fyi.uwex.edu/weedsci/2001/07/04/wild-parsnip-3/
It can lead to some SERIOUS burns. I'm not going to post the pictures but those of you with a strong enough stomach can follow this link.
https://www.google.com/search?q=poison+parsnip+burns&num=100&safe=off&tbm=isch