This is a tough call. Russ Thomas may have been the most irritating, but Russ was actually succeeding in doing what he and WCF wanted to do, which was keep enough flash (Owens/Sims/Sanders etc) on the team to keep 'fannies in the seats' without he or WCF ever having to work hard at the actual details of team building. So his failure was as much by design.
So it comes down to Millen vs Weaver - who have in common that neither had/has the slightest clue how to build a team for the league their team is in, always a solid 1st step toward ignominy for anyone arriving at a GM gig. Millen of course has the longer tenure to his (dis)credit, but three of Millen's teams were merely mediocre as opposed to terrible. Weaver OTOH, took a bad team and then managed to take it the rest of the way to terrible and then made it incrementally worse each year since, while simultaneously gutting the team's prospects to ever get out of the hole he has dug. This combination of feats, in the end, stands alone and above all the other GM ineptitude in Detroit history despite his having achieved it in a mere 4 yrs.