I'd argue Don Waddell is the worst GM ever, even ahead of duncelords like Milbury. He is a major factor in how the Thrashers moved to begin with;
He was the first (and only) GM of the Thrashers. In that time span they made a grand total of 1 playoff appearance and only finished above 3rd twice (both of which were because the Southeast division was brutal)
in 2007, they won the Southeast division and gearing up for a playoff run, Waddell shipped away significant future assets. The most notable getting an already declining Alexei Zhitnik for Brayden Coburn. Coburn would go on to be a staple on the Flyers blueline during their last run of contention (2009-2012) while Zhitnik was largely ineffective for Atlanta
Perhaps even worse, he shipped away an entire draft for multiple picks. The result? Atlanta gets blown away in the first round by the older, but much better constructed Rangers
He lost several key players for what amounted to nothing; Marc Savard was allowed to walk in free agency, Dany Heatley was so fed up he demanded out and got shipped for Marian Hossa, who also was traded for nothing not even 2 years later. Ilya Kovalchuk was traded after he rejected a hilarious 12-year, $101 million contract (somehow not the most insane deal Kovy would be offered and a deal that would have also killed the team long after they left had it been signed)
Another staple of Waddell's time as GM was his fetish for rushing draft picks to the NHL. most of which blew up in their faces In his last 3 years alone:
2008: Drafts noted IR regular Zach Bogosian 3rd overall. He plays in his first season and looks okay at times due to restricted minutes. Come the next two years he's assumed to be a top pairing guy and his play takes a nosedive from the extra responsibility, going a -45 (i know lol +/- but still) in the next two seasons. Rest of career is just missed potential and injuries. The guy that went after him? Future stud Alex Pietrangelo
2009: Selects Evander Kane 4th overall and throws him into the deep end right away. To Kane's credit, he looked like he belonged, posting a respectable 26 point season as an 18 year old, and would then better his goal total for each of the following two seasons. Also became a fan favorite to everyone outside of Atlanta after he utterly dumpstered Matt Cooke in a fight.
2010: Probably the worst all; picked Alex Burmistrov 8th overall and had him penciled in from the getgo. Burmi being a small 18 year old looked lost in the show at times, especially in his own end. Never really had the chance to develop properly to the point he's barely a KHL player at this point.
Those are just a smidgen of Waddell's draft record...the rest? Not very pretty. First round picks:
1999: Patrick Stefan (massive bust, but that whole draft sucked)
2000: Dany Heatley (successful, then decided to wrap his ferrari around a tree and was traded)
2001: Ilya Kovalchuk (obvious #1 pick, no credit here)
2002: Kari Lethonen (decent goalie but hardly worth 2nd overall pick)
2003: Brayden Coburn (good, but never suited up for Atlanta)
2004: Boris Valabik (bust)
2005: Alex Bourret (bust, never played a single game in the show)
2006: Bryan Little (decent, but also had a long period in ATL where he was ineffective because he was bounced around the lineup/had no talent around him)
2007: No first round pick because Keith Tkachuk presumably ate it.
2008-2010 already mentioned
His drafting outside of the first round is also pretty poor. The only major gems he found in his 10+ year tenure were Tobais Enstrom in the 8th round of 2003. Literally no other players he drafted became significant contributors.
It was that combination that resulted in the Thrashers being garbage for 85% of their time in ATL, and making all the more easy for True North to swoop in and move them. The Thrashers system was so bad it took even the new blood in the Jets FO years to bring their farm system and actual roster to respectability