Jean should definitely be the second in command for the Rivaille squad (Levi Squad), he is the highest ranked in the 104th recruits being in the "Survey corpses" (I should stop playing with the names of the characters/organization in Shingeki No Kyojin).
Correction : highest ranked recruit with still his head on his shoulder and without any \olo/-super-power or really special role to fulfill.
Eren : Anger problems, usefull to plug holes in walls, punch himself in the face, control titans, really tasty... "Best squad leader of all times" (Mina and the others are thanking you). Goal of the squad : Protect him, would be strange if he was second in command. 1 titan kill as a human, a mountain of titans as the "Rogue Titan".
Potato Springer : doesn't understand the concept of strategy, good soldier and support for a leader but not a leader. "1# World genius" Not a single confirmed kill, maybe with Historia in chapter 50 and a few assists ?
Sasha : a great asset for a group, she has the same type of intuitions as Mike, may be really usefull if they want to make the right choice, remember the forest ? "Potato Girl". No titans kills confirmed, but the bow and arrow scene...
Historia : she knows some stuff, she is nice to everyone, she may help to hold the group together, a bit like Yoga Ral, I mean Petra Ral... Plus she is important they cannot have the person they need to protect as a second in command. "Eh ?" At least one confirmed kill, maybe two or three with Ymir and Connie.
Mikasa : the most likely to be the second in command, she already worked with the smallest strongest man in the "Cleaning Legions", she may learn from him, and well she fits the job (protect Eren and Historia, but well she wouldn't give a single duck about Historia because "Eren !"). She already took the lead of the operations in trost (for the bad reason), and she is really popular amongst the fans (but the author is not really caring about the fans in general), it would be logical if she was second in command BUT i wouldn't enjoy it. Between 5 and 10 titans kills...
Armin : Excellent strategist, proved himself usefull a lot of times, but doesn't have the shoulders to be a leader in my opinion, he can be commanding troops, but not leading them... "Mini He-Man" Direct Titan kills : 0, but it's not confirmed; number of Titans killed with his plans : 7 plus all the titans outside of the supply depot, and Annie captured thanks to his magnificent brain.
Jean : Not too bad at strategy, really good with the 3DMG, knows what it is to lead people and the weight of it on his shoulders, doesn't want people under his command to die anymore, can take risks himself if there is the need and had a character development going in this direction. BUT (with one T please), he only interacted with Rivaille twice (anime only), so I would like to see Jean becoming the second in command. And he survived against the Female titan while fighting it and not being a titan shifter... (in the manga, there is only the scene where he is saved by Armin, but in the anime he goes against Annie a few times with Armin). Plus he is one of the few having a confirmed titan kill, and it's without the 3DMG... At least one titan kill, plus a few assists and maybe a few kills if you think that in the "Jeanservice" of the anime he could kill some titans.
I know that Eld Gin said that a true soldier is not recognized by his titan count but it may help to see if they already kicked some titan butts.
Bonus : I want to see Jean be the second in command for the fact that he is a really direct person, and that he is clearly not caring about the cleaning of the cabin.
