Dandelions, in every stage of development from bud to fluff.
Thistles, though I usually catch them before they reach this stage.
I know oregano is technically not a weed, and I really do appreciate the first 27 clumps of it. After that, it's a weed.
Horsetail ferns, which are a permanent fixture around the pond and at the back of the property where it's wet. There is absolutely no way ever discovered or invented to get rid of horsetail fern. It will grow through landscape cloth. It will grow through plastic. It will grow through six inches of gravel, plastic and landscape cloth. You have to admire that kind of persistence.
And I have no idea what this is, but it keeps growing in the beds along the perimeter of the pond. It starts off in a nondescript weed-like way. Then it erupts like a party favour in a giggly mass of tendrils and tiny pink flowers. It's so good-natured it usually wins a reprieve.