What was Walt's worst, most critical mistake? MAJOR SPOILER ALERT

The whole series is a never-ending chain of mistakes, accidents and poor judgement calls, but which of these was the most devastating?

Not including his original decision to get into the meth business or pre-show incidents like selling his share of Grey Matter, what is the most crucial mistake Walt made in the series?

My picks are these:

#3 Getting drunk and telling Hank that the mastermind of Gus's operation must still be around after Hank had given up on the case following Gus's death.

#2 Leaving "Leaves of Grass" with the "W.W" inscription from Gale in open view in his bathroom for Hank to find.

#1 The magnet plan to erase Gus's laptop. Not only was it completely unnecessary, as the laptop was encrypted, it directly led to the discovery of Gus's secret financial accounts, which caused the involvement of Madrigal and all the horror and death that occurred in Season 5. Of course, Walt & friends had no way of knowing the laptop was encrypted, but their decision to use the magnet is the key error that led to the "End Times" as Saul would put it.

Actually, the very, very worst mistake made in the actual time-frame of the show was probably turning down the generous offer to return to Grey Matter, but of the errors made in the actual course of building the meth empire, I would definitely choose the 3 I listed.

Note: My son and I still have to watch the last 2 episodes tonight, so I won't check this thread again until after then, so I don't get spoilers in case Walt makes even more crucial errors in those.