Theirs, and all of the other animated-image adverts.
Back in the late 90's, I learned an amusing phrase for these animated .gif files: "Dancing Baloney".
What annoys me, at the risk of being seen as a whiner, is that the "Dancing Baloney" is always the first to load, and starts its "dance" before anything else is allowed to load. Which, of course, slows any subsequent downloads.
My nominee for the worst such: those eye-busting, headache-inducing, flashing "Winner" ads.
Any other nominees?