Could be anything at all, something trivial or major thats annoyed you or made you smile for years.
Socialist wage policy? Signing Sanogo, Gervinho? Zonal marking?
Thierry? The invincible season? Pushing Mourinho? Vieira?
For me there have been many.
I could never and still to this day cannot accept nor understantd his neglect of the goalkeeping position and defensive tactics and training in general. How many points/trophys this has cost us we'll never know.
On the converse I dont think Ive ever seen a better midfield partnership than Vieira /Petit. Ever. Just a joyous thing to behold.
So post your views on Wengers best and worst decisions before Quartz deletes/re-names/moves/locks this thread.

So far we have.....
Subbing off Bobby after Lehmanns sending off. CL Final 2006
Dropping an on-fire Arshavin for 2009 FA Cup semi.
Not activating Cescs buy-back clause, allowing him to go to Chelsea 2014
Winning the PL for Man U by agreeing to sell them RVP after speaking with Fergie 2012
Decision to implement a "socialist wage policy" leading to hugely inflated wages for mediocre players
Selling Ashley Cole to Chelsea under "socialist wage policy"
Man management of Vermaelen, Podolski and now Ozil
Allowing Pires to go too soon under one year only contract offer policy to players of a certain age.
Decision to field teams littered with "projects" for importnt cup games, eg 2007 League Cup Final
Decision to sign a new contract when he should have left after FA Cup win 2014
Persisting to play a shit comedy Eboue 2008
Passing on opportunity to sign Yaya Toure
Giving Djourou a new improved contract 2012
Persisting with "world class" Almunia for way too long and making him captain
Losing Vermaelen Sagna and loaning Carl Jenkinson leaving us terribly vulnerable at the back 2014
Good transfer decisions eg good money for Merse Song, Ade and nabbing Vieira Henry proved fantastic buys
Persisting with Rambo when it looked like he wasnt gonna cut it