Good thread.
TLDR summary of serious contenders (overlooking sarcastic answers).
(remember don't shoot the messenger - I've just compiled, I'm not endorsing)
Ana - Randy Caralyle, Dallas Eakins, losing Selanne
Ari -
Bos - 2015 wasted draft; trading Ken Dryden's rights to Montreal;
Buf - Pegula ownership; "we invented decade of darkness before the Oilers"; Drury over Briere/ not prioritizing player who wanted to be there;
Cgy - underwhelming return on Phaneuf tradel losing Martin St. Louis; prioritizing Trevor Kidd over Martin Brodeur; Gilmour trade; how did nobody mention losing overweight little kid called Brett Hull?; Jay Feaster hire;
Car - wrongfooting themselves with new fanbase by starting out in Greensboro
Chi - trading Hasek; overpaying Seabrook; trading away unproven Phil Esposito
Col - general drafting missed opportunities; not building around ROR
Cbs - Gilbert Brule flop
Dal -
Det - trading away HOFers Hall, Bucyk, Lindsay, Kelly
Edm - trading Gretzky/ allowing Pocklington to use franchise as ATM; Peter Chiarelli era (still paying dividends even today!)
Fla - Paul Maurice; ownership meddling during Tallon era; Tom Rowe episode; losing Marchessault for nothing
Hfd - killing the franchise (allegedly) by trading Ronnie Francis
LA - Lucic trade; losing Cernak; trading away too many picks to chase short term returns;
Min -
Mon - Houle/ Tremblay era; not tanking; the way Roy left town; moving on from Koivu, McDonaugh; Marc Bergevin (really? worse than Houle/ Tremblay?); Ron Corey
Nas -
NJ - losing Lou Lamoriello (rebutted by subsequent posters - bigger error is allowing Lou never to tank)
NYI - losing Tavares for nothing, Spano debacle or Mike Milbury, not sure which;
NYR - 2003 draft missed opportunities; trading Rick Middleton; not drafting Mike Bossy; multiple crippling trades; not capitalizing on 1994 window;
Ott - Brian Lee busted home draft; Redden over Chara
Phi - couldn't Lindros trade be reworked to not lose Forsberg; not signing Curtis Joseph; Nolan Patrick over Makar, Pettersson
Pit -
SJ -
Sea - you're not old enough to have bad moves yet.
StL -
TB -
Tor - trade for Kessel; trading Rask; trading Niedermayer pick; Harold Ballard multiple disgraces; losing Lanny McDonald; Brian Burke era;
Van - Jim Benning hire (OEL, Loui Eriksson, Tanev etc are all subsets of this); Aquilini using franchise as plaything; Mike Keenan; Mark Messier; not drafting Kopitar;
VGK - loss of Suzuki/ prospects to chase a championship which didn't come
Was -
Wpg - too much faith in Pavelec; too much faith in Maurice;