wow

Well as a weed is simply a probably invasive plant growing in the wrong place I guess it depends to some extent where you are. I am tempted to suggest sycamores for Britain!

For the Azores, it would have to be the Eucalyptus. It is listed as 100% invasive on the biodiversity site and people are banned from planting any more of it. That is a vertical giant.
Area-wise, throw a piece of kudzu somewhere it likes, and there will be a problem.

Here in the Valley of the Sun, it is probably White Popinac (Leucaena leucocephala). Grows 60 ft. tall, seedlings everywhere, and root sprouts, too.

Sounds awful, Tb. I agree with Stera about the Sycamore here in the U K.

By Sycamore, I take it that you mean the Sycamore Maple (Acer platanoides).
Wylie may have the title for the largest weed, though. Eucs can get more than a hundred feet (30 m) tall, with trunks more than 5 feet (1.5 m) wide. Here in the Valley, we're grateful that they rarely seed themselves, unlike in the Azores.

I can't stand crab grass. Granted they arent the biggest but just a few can ruin your lawn and make the whole place look tacky. You can get a hernia trying to pull them up and just one plant can produce over 70,000 seeds. It isnt edible either like dandelion

depends on what you mean by biggest. height width spread, ability to spread [like the crab grass etc]
more info re what you mean by biggest.

I think the purple rhododendrons growing out of control in areas such as Snowdon are causing quite a problem. (Don't know their Latin name.)

Acer pseudoplatinus is what I meant Tug.

Hah! I was close, but no cigar--just as well, since I don't smoke! :]

A miss is as good as a mile - glad you avoid the most noxious weed of all, if not the biggest!

I don't know about that, Stera. We have both yellow and purple nutsedge, which are impossible to get rid of, once you have them, and Bermudagrass is a horrible weed, even if it is our best lawn.

Tobacco does more damage to folk in the long run...

Something to be grateful for that we don't have the three plants you mention though! I just looked them up - the Bermuda grass certainly looks a tougher cookie than our lawn grasses! I see the other two are sedges - I thought most sedges likes damp soils but that can't be true!

Ah! Sorry it took me so long to twig to what you meant by "weed"! :)
These two sedges are from India, and while they prefer moist soil, they will survive years of drought, waiting for lawn watering, or some nice, moist rose beds!

I am plagued by Bermuda grass. It started two houses up, and grew into the stone walls (which makes it really tough to kill). Then across my immediate neighbors yard, and is now trying to invade my place. The smallest piece left behind will grow quickly. I also have buffalo grass trying to invade. They are really good grasses if you want to grow something for the cows to eat, not so good for a garden.