IMO, the Milloy departure is still the biggest shock of them all (Branch, Seymour, Moss, Mankins, etc...).
That one really came out of nowhere and it was still rather early in the Belichick tenure (one SB title at the time) and he hadn't done anything on the scale before. I mean everyone knew the Bledsoe trade was coming the off-season before but the Milloy thing was just absolutely stunning.