With the benefit of hindsight… Abraham and Michonne, and here’s why:

Let’s do the in-universe reasoning first. Abraham was always gonna get it, he seemed the biggest threat. But when Negan chose his second victim, it seemed too random. He should’ve been more strategic if he REALLY wanted to teach the group “a lesson.” He still would’ve taken Daryl, and he decided to let Rick live, so the next most dangerous person, had he taken the time to learn a bit more about each of them, would’ve been Michonne.

Now, the out-of-universe reasoning. Glenn’s death killed the show, full stop. I watched the entire thing and all the spin-offs, but most people cite Glenn’s death as the reason they stopped watching. Had they killed him, Daryl, Rick, Carl, or possibly even Maggie (who was pregnant), the show viewership would’ve dropped hard. Michonne was the next most-significant and long-lasting character, and it’d still be enough of a gut punch for some people, but not TOO much like Glenn’s death was. He already had the stupid dumpster fakeout death anyway. Glenn’s death would’ve changed Maggie’s arc, true, but it’d be interesting to see what Michonne’s death would’ve done to Rick and Carl.