Glenn, for many reasons.
In my opinion, Glenn's storyline has reached its end. He's been married to Maggie for the last three seasons. He was separated from and reunited with her at least three times (journey to Terminus, the walker parade plan going awry and almost dying at the dumpster, and Maggie being kidnapped by the Saviors). Maggie is carrying Glenn's child--his legacy. Glenn finally got his first human kill(s) this season. I don't think Glenn's story has anywhere left to go.
The whole point of Glenn getting that death in the comics was that it would not only severely affect the characters and drive the story forward, but completely shock the readers. His demise devastated every character who witnessed it and left the readers reeling. The same circumstances apply here. Of all the characters in the Lucille lineup, whose death would deeply affect the most characters? Rick, Carl, and Daryl have known Glenn since the beginning. Glenn (accompanied by Tara) was the first character from the core group that Abraham, Rosita, and Eugene met. Maggie...obvious reasons. Aaron became close to Maggie in the first part of the season, so he would be affected by Maggie's devastation over Glenn's death. Michonne and Glenn have had a lot of screen time together this season. Every single character would be losing a good friend/spouse who is, without a doubt, one of the most likable characters. As far as the audience goes, Glenn's death would be enough to hit the viewers hard in the feels, but not so much that it would threaten the loyalty of a huge number of viewers who love a certain character (cough...Daryl). The bottom line is that this particular death is meant to propel the story forward and hit the viewers hard, and the death of Glenn has the greatest power to do both of those things. Not Daryl. Not Abraham. Not any of the more minor (but still important) characters, like Aaron or Eugene.
The show has often switched up the deaths to keep comic readers guessing, right? Most of the deaths in the show were either (1) taken from the comic but given to a different character, such as TV Hershel being given comic Tyreese's death, (2) remixed from the comics, such as Billy and Ben being gender-swapped to Lizzie and Mika and basically being given the same deaths, or (3) completely original or different, such as Lori's death in childbirth. I've seen so many people predict that it won't be Glenn, since he died that way in the comics and it would be too obvious. I feel as though people are so convinced that Glenn won't die, that killing him would actually be the most shocking thing the show could do. So few characters in the show have died exactly as their comic counterparts did that it's unusual when it actually does happen. The idea of Glenn receiving his comic counterpart's death is easily dismissible for being so predictable, and therefore it's totally unexpected by those who are so convinced that it won't happen.
Then there's all of the foreshadowing throughout the past two seasons. Glenn almost got clubbed with a bat at Terminus. He picked up and held a bat at Shirewilt while he was talking to Rick and Michonne in the driveway. He saw the polaroids of Lucille's victims. Maggie mentioned a price to pay while they were at the Hilltop. All of this foreshadowing could be a true indication of Glenn's death, but it could also be intentionally making his death seem so obvious that it could throw people off and make us think he won't be killed, only for him to actually be killed off. Also, Glenn has had three dangerously close calls since the Season 5 finale (shot by Nicholas and attacked by walkers, the dumpster fake-out, and getting swarmed in No Way Out). It seems as though the writers are using all these close calls to lead us into a false sense that Glenn is unkillable. All the more reason to kill him off for shock value.
All of that isn't even taking into account what happened in the finale episode itself. I think Glenn sealed his fate when he stood up to defend Maggie. In the comics it appeared as though Negan chose his victim at random, but in the show I got the feeling that Negan had already chosen his victim and played the eeny-meeny-miney-moe game just to screw with everyone. After all, it looked like he started skipping around near the end of the chant. Glenn's outburst had good intentions, to defend Maggie, but it also revealed that Glenn and Maggie were in a relationship. The look that Negan gave to Glenn, an evil grin that had a "well well, what do we have here?" look to it, said it all. I think Negan knew that by killing Glenn, he'd devastate Maggie, effectively destroying two people at once. Also, I'm convinced the POV shots throughout the episode were the clue that was mentioned on Talking Dead, allowing us to narrow down the victim to the four captive characters in the van, one of whom was Glenn. Those POV shots have to mean something or have some significance. Last thing I want to say...a small thing that I noticed was that Glenn was the first character whose face we saw after Negan "introduced" Lucille.
And now that Steven Yeun has a new movie coming up, I'm even more convinced that Glenn is the ill-fated character.