The Daily Grind: What makes for the best in-game events?

It's several years now since the great Rikti Invasion that ended the City of Heroes Beta, but people still talk about it. Vast ships filled the sky above the city zones, and huge spawns of Rikti beamed down to attack the player heroes - led by developer-controlled bosses.

There have been other events since, but it seems nothing has ever quite lived up to the impact of that first event, possibly because it's now passed into game legend, and has been told and retold so many times it's become more than it was. That aside, we can identify three aspects that made it epic: big, obvious additions to the regular scenery; frantic large-scale action across a wide area; and active 'event crew' participation, adding the human element.

Which in-game events have been the most memorable for you? Have you interacted with signature characters or taken part in one-off sieges? Have you attended weddings that turned into slaughterhouses, or invasions that turned into mass dance parties? We'd like to know what you think makes an in-game event truly memorable, no matter whether it's arranged by developers or players.