Annihilation takes the prize every time, for its sheer epic scope.
Secret Wars 2015 is my favourite superhero event.
Civil War for its importance to Marvel history both in universe and in publishing history.
I have a soft spot for a couple of less loved events. I loved AvX when most seemed to find it cynical. I loved the way it developed and its non-straightforward, messy resolutions. I loved Fear Itself, again for its scope and its final battle.