Well, based on that video... (speculation in spoiler)
Spoiler!
I'm thinking it's Abraham. The blood spatter on Rick's face seems to indicate that it was someone sitting close to Rick and on his right side. Also, the pool of blood at the end looks like it's close to where Rick was sitting.
Maggie is directly beside Rick and Abraham is right beside her. I don't think Negan would kill a woman like that, so that makes Abe the most likely based on proximity.
Glenn is on the far end of the line. Daryl is pretty far from Rick too, on the other side of Michonne.
Although... Looking at it again, when the camera tilts down to the blood, you can see a pile of fabric on the ground. The only person who was wearing something like that was Daryl.
Everyone else is wearing a jacket that wouldn't have fallen off during a beating.