It's Granlund and I don't think it's particularly close (though people going off menu for Brassard might be onto something)
Reasoning:
JMFJ and Reaves were bad moves made for ... understandable reasons. (I don't even hate Reaves as a player, he's not a goon, he's a legit NHLer)
The Pens needed defensive help. Johnson has blackmail on every GM in the league. So like, it "makes sense." Johnson is just far far faaaaaaaaaaaar worse than anyone in the NHL is willing or able to admit.
Reaves, again, filled a perceived need. It wasn't a need. But in NHL circles who aren't as intelligence-forward as you'd like, it was a need. And as far as "muscle" goes, Reaves isn't a goon, so he was probably the best combination of: skill, toughness, affordability we could find. That said, the price was still absurd.
With Granlund, the Pens weren't one mediocre 3rd liner away from contention. The season was clearly going to be a loss, but they had something going for them: a shit load of salary was either off or coming off the books this off-season.
prior to Granlund, it was something like 20,000,000, i think. And all they had to do was find a goalie, a defender, and maybe a top-9 forward.
It was the most-healthy the team has been from a cap perspective in years.
Not only did they kill a ton of that flexibility, THEY GAVE UP A SECOND ROUND PICK TO DO IT.
For a player who is not good. He's, at best, a 3rd liner. You can find 1,000,000 granlund's on July 1st. And you'd likely pay him less. And you'd save the pick.
Just an absolutely unforgivable move. It showed a lack of understanding of what your current team is, and a lack of vision for where it could go next season.