Is that a trick question ? Onslaught was worst than everything that ever happened before it. I wouldn't call it the beginning of the end because, Age of Apocalypse aside, things were already rather dire but it was the final nail in the coffin and killed lots of fans enthusiasm for the X-Men. Cross-over fatigue, nonsensical plot and a billion tie-in books. BOOM.
All truth! Covers all of my points, except to say that it was at least refreshing to get over a half year of teasers about something instead of it just being dumped on our lap.