Title says it all. List as many as you wish.

Worst: Rickrolling.

Rickrolling has its origins in a far older internet phenomenon, called duck rolling. This involved placing a blind link in an online discussion. The unaware user, keen to follow a strand on their subject of choice would click the link and end up visiting a page that had nothing to do with the subject at all. In fact, the link would generally lead to a page the contents of which would be literally a rolling duck. This became known quickly among internet wags as duckrolling.

So, things evolve. Instead of seeing a rolling duck, those who are now “rickrolled” find themselves on YouTube. Rick's 1987 hit “Never Gonna Give You up” has now received more than twelve million hits. Not quite the revival he was looking for, I would guess.

It started on the day that “Grand Theft Auto 4” had its web premier. The traffic to the host website was so heavy that most people trying could not watch the trailer. An anonymous prankster took it upon himself to link everyone elsewhere. Under the guise of redirecting people to a leak of the trailer on YouTube, Mr (for it surely has to be a male!) Anonymous instead linked the Grand Theft Auto gamers to Mr Astley and his chirpy 20 year old hit. Why he chose Rick Astley will remain a mystery forever, or until the culprit is brought to justice and forced to sit in a darkened room listening to Barry Manillow for several months.
Very annoying.

Best: I'll list one when I can think of one.