I don't think we have to count Erwin out just because he lost an arm. We actually have never seen him kill a titan anyway, and he hasn't needed to command up until now. His job is to think two steps ahead and maneuver politically. Trying to slice up titans would only be a distraction while commanding a battle formation. At most, the physical requirements for him are to ride a horse and use 3DMG. He's not done yet.
But I think he will die soon. He's made too many gambles, and one is bound to catch up with him. I think he will get to a point in the story where he learns why Reiss is doing what he's doing and die almost immediately after. The only thing we get to see are his facial reaction, and a bubble from Reiss saying "and that's why this is necessary. After he dies, the Scouting Legion will dissolve and a few rebels from the 104th will probably follow Jean. That's what this has all been leading too right? Jean taking command?
IF the Scouting Legion survives though, I vote for Levi. When Erwin was out, didn't he already say that he issued orders, including reassigning the 104th to his own squad? Hanji CAN do it. I've been impressed by her abilities during the "kidnap the 1st MPs" mission, but I think the troops would get nervous following a known whacko. Command presence and trust is important, and by that measure Hanji is out. Armin too, since he's too green.
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More recent ones: when Ymir took Historia in chapter 47, and chapter 50 with Armin. He's the one who was "giving orders".
I like the way Jean knew exactly how the Female Titan would disrupt the formation, and realized that they might have to sacrifice their lives to prevent the SL's total annihilation. The other two were just staring at the "Megata's" nice ass, but Jean understood how the formation worked and decided on a course of action. He also figured out the point of the mission along with Armin (hanging out in the forest). He's shown a keen mind for tactics, operations, and strategy so far.