Nightmare Weed

All right, I'm curious. I am engaged in an ongoing debate/dialogue/cursing fest with myself and coworkers regarding which of many is the worst weed/invasive species to remove. I tend to go back and forth between Oxalis (oh those pretty little shamrocks from hell) and bindweed (Convolvulus arvensis, whose explosive germination any time earth is moved from one spot to another seems to support the centuries-old theory of spontaneous generation).
Less common, but more likely to spawn genuine nightmares is Alstroemeria. I kid not, after a whole afternoon of digging and searching for these elusive, grave-depth fleshy tubers I had a restless sleep in whose dreams I was likewise digging endlessly, but with my spade repeatedly striking and uncovering unexpected objects like full bottles of insecticide spray. This was disturbing in a way that I can't quite justify or explain. Last week I returned to a garden from which I meticulously removed every last trace of Alstroemeria to install some plants. Lo and behold, when digging holes at a depth of 12-18", I began to unearth those wormy white clumps that would be next years crop. For all I know, these tubers penetrate the Earth's mantle and can be found well into the core of the planet; I am afraid to ask whether oil-drilling outfits have contingency plans for encountering abnormally large Alstroemeria clumps at depths over 2 miles. I can no longer simply smile and compliment someone's lovely bouquet of store-bought Alstroemeria without choking back a grimace and repressing a handfull of insidious memories.
Thus, after some delay, to my aforementioned curiousity. I would like to know what other people's nightmare weeds are, and for what reason. Please feel free to leave your opinions in the comments section. If I get enough different responses, I would like to put up a poll so others can vote on their least-favorite weeds.