Most decisions since the latter years of Brees / Payton. There's no need to elaborate on it as it is just a summary of this forum of the last 5 years or so.
If i had to pick the 3 worst decisions in order of magnitude desc:
1) Poor Drafting - picking the players (which is a lot of decisions wrapped in one)
2) Hiring Dennis Allen
3) Draft pick trades - again. lots in one