I haven't commented for a while so just going to belatedly give my thoughts on the finale after digesting it and reading various theories.
First - the episode itself. It mostly felt like contrived setups, running around in the dark, and padding out the episode with them putting everyone's head on the chopping block before the inevitable final scene which everybody knows is coming...which the writers know they know...and so we have this playing with the audience throughout. The final scene in the woods was pretty well done and chilling with all the whistling but I didn't quite buy how broken they all seemed. I think they should have had a more difficult journey to that point and more deaths (I always want more deaths!) rather than just being broken through helplessness.
As to who it is.....no idea really. I watched various concocted theories on youtube. Half a dozen sound semi-plausible. Half the stuff they rely on could just as easily be production inconsistencies.
Seems plausible
Abraham - based on the scene it would make some sense, assuming Negan is in anyway in control of who gets picked (which is iffy because one time he might have 11 people in front of him and another he might have 25, or 5, or 43). But assuming he was skipping around the order a bit and he wanted to make an example to the group then it would make sense to batter the biggest person there and the one who seemed the least afraid. He also seems the most likely to take a blow and keep moving - if hard-headedness is anything like being massive.
Glenn - Dead, not dead, dead. Seems the ironic kind of thing they might do. Plus
- which is as valid a reason as any. Plus the POV shots which seemed to indicate him, Rosita, or Michonne.
Aaron - Couple of reasons. They mentioned him feeling like he owed Maggie. Could be suggesting he's going to make a sacrifice. Also is not as central to storyline and they do seem to have laid off killing major characters recently. And I had a go at matching up the scene with the rhyme and what was shown before the order got messed up, with the syllables used, and got Aaron.
Eugene - Had a few scenes early in the episode doing the usual redshirt thing of sharing knowledge, which often signals a soon-approaching death, so maybe. Also has seemed a bit peripheral recently.
Daryl - I would have thought it more likely him before the bit with him getting shot and stuff. I think him vs Dwight will be next season. Still possible.
Unlikely
Rick - It would be a ballsy move. There were rumours last autumn, that were quickly quashed, that the actor wanted to return home. If GOT can kill their main character then I guess another show could try it. The scene did suggest it wouldn't be Rick or Carl though since the "cut it out and feed it to his father" line did seem to be spoken by Negan as he stood in front of the eventual victim.
Carl - Again, it would be unexpected. I doubt they would though. Also - see above.
Rosita - Having watched a few theories it seems as though people are saying that Negan's comic book counterpart wouldn't have picked a woman or child as his victim, but, as far as I'm concerned, the two (comic and tv show) are different entities, so I don't think the tv show Negan has exhibited any protective feelings towards women yet. She was also in the van so the POV could be hers. I don't think it will be though. She had no real dialogue or last scene you usually get put in for dramatic effect, foreshadowing her exiting.
Michonne - Also fits the van POV shots but I don't think they'd willingly take the character out of the storyline at this point unless the actress wanted out.
Maggie - I doubt it really. Wouldn't have got up after the first blow in the physical condition she was in at the time.
Sasha - Nothing to suggest her that I can remember.
So I'm sort of between Abraham, Glenn and Aaron over who likely gets the bat. Rick, Carl, Maggie or Michonne would shock me the most. I guess we'll know before the new season airs with leaks from shooting or even people putting two and two together seeing what new roles actors are signing up for.