Firstly, I listed a few, but I see you're looking specifically for transactions, not where he(Dubas) went against the opinion of others.
Trading good assets, whether 1st round picks, or good prospect is seen as going all in to a degree and this year teams went crazy.
Keeping the core four together, when they should have been dealt at the latest of last off season is.
Keeping Keefe, who should have been gone easily at the end of last season is another.
Neither one of those are "transactions", so I'm guessing that means they don't count.
Trading Mason Marchement, who was showing well that season in the AHL for Denis Malgin was a "smartest man in the room", or just plain stupidity. A player you need, who would look good in the bottom 6 was dealt for another soft player your deal had absolutely no use for.
Trading for Matt Murray, a player everyone knew sucked and barely made Ottawa pay is a "I'm the smartest man in the room" move.
Dubas had to make the move of dealing a 1st (Seth Jarvis) to rid themselves of Marleau because of the cap.
Looks worse now, but keeping Kerfoot over McCann
Traded for Rittich, which is likely more just a bad move overall, so I wouldn't say that's the smartest guy in the room deal, but it's just a bad one, but doesn't hurt as he wasn't expected to be the main, or one of the main goalies.
Signed Petr Mrazek, who everyone knew wasn't that good and injure prone, cost the Leafs and cost them to move back in the draft.
He signed and later traded Matt Martin because of cap issues and due to those cap issues, brought in and kept Kyle Clifford.
Tavares signing wasn't really a "smartest guy in the room" move, but a lot of people questioned if the Leafs actually needed him and the answer seems to be a very strong no.
Just because other teams were gunning for him, doesn't mean it's a good move.
Not entirely sure if it would have happened, but the Leafs might have been able to acquire Lindholm and sign him instead of having Tavares.