It can't be Eugene because of where they were kneeling
Eugene gave up the bullet directions and Abraham knows where to get the supplies. Could point to Eugene.
Eugene was on the end to Carl's left or Negan's right while he was facing the group. When Negan is standing before the vicitm, he mentions Carl and motions to his right towards Carl. If Negan was standing in front of Eugene, he would have had to motion the other way for Carl.
I think it's Glenn. His death plays too much into the story. My second guess is Abraham. They really played up his choice to start "Living". Third guess would be Daryl simply because they said it was a beloved character.
I really think we got robbed of the most pivotal kill of the series. They should have shown the full violence of Negan by showing every swing of the bat and crush of the skull then showed the victims body crumpled on the ground and faded to black with the screams of the other characters in the background. That would have had a HUGE impact.
They really watered down Negan's vocabulary and they hurt his character. Take away the brutal kill and Negan really doesn't seem as bad on TV as he would have with the language and the skull crushing kill.
And actually, their cop out of a cliffhanger really left us not knowing for sure someone actually died. Yes, we believe someone did because that's what happened in the comic and because some guys said so on an after show. But Negan did say he didn't want to kill anyone and he said he was going to be the holy hell out of someone but he never said he was going to kill. So, it could be another "Glenn under the dumpster" deal. We know it's not but that option is still there. Had they shown the kill, we would have no doubt and we would all hate Negan!!