OK now here is a fun question with a poll attached.
Now there certainly have been a lot of events that had negative impact on the X-Men. House of M certainly comes to mind.
But from at least from my view there is one which really stands out in terms of being disliked and having a really negative impact on the franchise and that is Schism. Since it lead to a split in the X-Men and from the look of the things even a split in the fanbase(constant bashing of fellow mutants and X-Men like Cyclops/Wolverine/Storm/Beast is yawn inducing at this point).
So is Schism one of the worst events and ideas to ever effect the franchise?