These aren't just the worst. These are the most worst. Chime in, add your two cents, and let the other dozen readers of this blog know how you would rate them.
1) Oxalis: If I were granted the wish to eliminate for all time one garden weed from my garden, it would be Oxalis. But if the djinni were to ask, 'which one?' I'm afraid I would be stumped. Both Oxalis stricta and O. dillenii are (apparently) native to North America, and it's hard to tell them apart. Michigan flora tells me that the former tends to have a single stem, whereas the latter is more branchy (my word). Anyway they are both ubiquitous weeds, and their exploding capsules reliably propel seed into every nook and cranny of the garden. These plants have the common name of 'yellow wood-sorrel,' and they can be mixed with salad greens to add a lemony spice.
3) Black medick (Medicago lupulina): with it's clover-like leaves and yellow flowers, this weed is probably familiar to anyone who gardens. You have to grab the stem right at its base to get the roots out. If you're in clay more drastic measures might be required. And the base of the plant always seems to hide behind or amidst the stems of those things you want to keep.
4) Hairy crabgrass (Digitaria sanguinalis): ignore the garden for a week, and the crabgrass plants will reach 10" across when you return.
5) Virginia creeper (Parthenocissus quinquefolia): presumably it was the wet spring that enabled the thousands of virginia creeper seeds to germinate in my garden. Even without the new seedlings, this vine is hard enough to keep out of the flower beds. This spring also brought an amazing number of oak and hickory seedlings. The oaks were easy to pull up. I'm afraid I'll never be rid of the hickories.
6) Asian bittersweet (Celastrus orbiculata): I'm giving this one a prominent place in my top ten list because it is the scariest. I've seen woodlands destroyed by this nuisance. It has a solid foothold in my neighborhood, and the birds are continually defecating its seed into my garden. Bad birds. There's no doubt about its identity when you pull out those reddish roots.
7) What? 10:00 pm already?!! Help me pick the final four!