I'm a very big fan of Erwin, and yes, Levi is one of my favorite characters (After Sasha, Mikasa, and Jean), but Erwin is my most. His character is complex, and even though he's quite ruthless, he has a human side. A very human one in fact. So I'd like to state why Erwin would've been a better choice in injecting the titan serum.
First off, I'm not going to say Armin is horrible, I really like Armin, but I have to say I'm heartbroken that Levi chose Armin over Erwin (Makes me like him less, I'm sorry). Sure, Eren said that Armin has done better than Erwin in the battle, sure, he suspected Annie, sure, he's highly intellegent. However, in terms of importance, Erwin is much more important than Armin. Don't get me wrong, Armin is very bright, optimistic, and yes, a great friend, but in terms of things like tactition, being a true combat leader, Erwin takes the cake. Sure, Armin can be smarter than Erwin, but not on a combat field. But I'm pretty sure in Chapter 84, after Armin eats Bertolt and when Hanji opened Erwin's eyes and confirmed Erwin's death (She didn't even go see Titan Armin. A titan-crazy person didn't go see Armin Titan.), it was the most emotion Levi ever showed in the series, meaning that Flocke, Levi, and Hanji has a lot of trust in Erwin. Levi knows that Erwin doesn't deserve to die, but deserves to have a good rest (THERE'S A DIFFERENCE I SWEAR)and to see all their other lost comrades (Like Mike, Petra, Marco, Nanaba, Moblit, and all the dead comrades watching from a happier place). On the part when Levi flashbacked to see Erwin thanking him, that was when Levi made his final decision. Back to the topic why Erwin should've been chosen. Erwin has a dream, like all the others, and all he wanted was to achieve it. I want to conclude this with; I'm not saying Levi made the wrong decision, I'm saying that Erwin deserved to have a good rest, and Levi made the right decision. Erwin is in a better place, and it makes me sad that we won't get to see Erwin for a long time, but hey, at least he made the young comrades happy, and he made Erwin happy too. As Hanji said; "After joining the scouts, these endless stories of parting began.". With an end of; Erwin isn't just another comrade. He's a commander. And Armin's survival will mean something, Erwin's death will not be in vain, this is a huge impact on the story as well. So to all the people who's gonna read this, please keep in mind that I don't dislike Armin, and keep in mind that Erwin's death was not meaningless. Thank you for taking your time to read my long blog post.