I myself dislike the idea of a Leader. Big time. Seeing how multiple people have different views on how to deal and treat with Samus, it would almost [i]never[/i] work and they could argue for days. I still maintain that the best choice is Cassius because he's the one who seemed more neutral in the big picture. 
Kaze has proved since the beginning that he doesn't care much about other's opinion on their shared quest as in he criticized those with different opinions then his, and constantly gave his best effort to have the most attitude as possible. He has the most experience in being a lone Mercenary, not in leading... that,s my opinion on that topic. 

Amber, in the end, would never propose herself and also proved she can be hot headed. In the end, she probably wouldn't care too much as long as she can do her research and follow the original Federation's goal of capturing Samus. If the Federation had chose it, she would've accepted it without question. 

I know that it's impossible for David to lead for the same reason as Kaze, because he constantly disrespect to the majority of the group and some real bad attitude. Kate has a soft heart, but she seemed kind of Neutral. Vekt didn't speak a whole lot. 

Make of it what you want, but it's how I currently see things. x) Could change through characters interactions later on. I still maintain that I don't see why we would need a leader in a hand picked professional squad. 

I'll roll with whatever's chose, though.