Not The "I" Word

It occurs to me that perhaps I've been unfair towards certain plants - let's not call them "invasive"- which display a certain spirited inclination towards reproduction. Via sexual or asexual means, these plants (let's not call them "invasive") spread themselves in an opportunistic fashion; the tiniest patch of bare ground given a why-not?-shrug and summarily inundated with cheap, plentiful seeds (I'm looking at you, little weedy violets) or subverted by runners and rhizomes with all the subtlety of a teenager's attempt at casual intimacy (that's you, bindweed).
But perhaps not all of these "opportunists" should be treated equally. Some are truly despicable (please refer to my Nightmare Weed post) but we should not let these cast a pall over all vigorous, sexually-enlightened plants (I'd still rather not call them "invasive"), some of which have undeniably positive aesthetic or structural qualities. Surely everyone has a few "self-seeders" or "naturalizers" (or whatever the euphemism du-jour is) which they are willing to unleash upon their garden with the full knowledge that many hands-and-knees hours will be spent in quarantine mode, futilely attempting to restrict them to one corner of one bed.
I realize I'm opening pandoras box here because in gardening, one man's treasure is another man's vile, loathesome weed the mere mention of which is wont to launch a tirade towards or physical assault of the mentioner. Therefore, I'll go first. These are plants towards which I will cast a blind eye in a client's garden, even if I have been more or less commanded to remove them. At the end of my day, there is just no cold ruthless blood left in my veins for these misunderstood gems (let's not call them "invasive", please).

• Centaurea montana - LSD flower, as I like to call it, as in God must have been tripping if he made this. Fellow Blotanists may recognize this as my "favorite flower". No matter how much these spread throughout a garden and how hard they can be to remove, I always have to stop and stare when they are in bloom. Like some cerulean, radial sea creature that grew a stem and joined the Asteraceae, I never get tired of them.
• Crocosmia x crocosmiiflora - I frankly don't care if these take over a bed; I just can't stir up animosity for something that blooms so brilliantly during a time of the year when many other things are tiring-out (here in the Northwest anyway). Plus, if you really do want to remove them, the corms pop out like they were just waiting for you to come along and grant them the pleasure of being ripped out of the ground.
  
• Carex 'Frosted Curls'- I was initially alarmed by how quickly these self-sowed in a bed that I planted. The alarm quickly turned to delight when I realized that a plant chosen for its unique cool-green color and soft, mounded habit made an even stronger statement in greater numbers (plus it allowed me steal one of the seedlings for myself).

That's all for now, let me have it if you hate any of these, but you have to give me some alternatives.

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.