My thoughts exactly buttermilk - which is why I think it's Michonne. Glenn would be too obvious (and he has a clear weak point that Negan could exploit in the future), Rosita would be too meh, and Daryl was lying down in the truck so the earlier pov shots couldn't have been him.
Plus killing off Michonne would be the most catastrophic for Rick and the group - she's Rick's partner, Carl's mother figure and the best fighter they've got.
I want it to be Abe though (and from Negan's point of view it would clearly be Abe who'd give him the most trouble).
I wonder if the show is going to pay the actor in question not to film anything else in May to October and just hang around on set occasionally instead. Otherwise we'll know who it is by the end of May (as they start filming s7 at the beginning of the month).