As much as the the monster Poison Ivy vines are a PITA...trail access, tree felling, food and bird poo seedling ammo, etc. ...I would give it second place only to american bittersweet for fall color and beauty;-) Just last year, we killed a huge PI bush that had a quagmire of 3"-4" trunks all grafted together forming a 10" trunk. Poor thing had only an old fence post to get it start some years ago. The whole thing was (is) 8' tall and 10' wide. It wasn't really in a bothersome spot and it glowed in the fall, but the fruit set on it was amazing. There is no way anyone is getting under it to cut the trunks. Herbicides to the rescue.