All Spartans in the Spartan Branch have the exact same rank. That rank is Spartan.

Apollo Anders rank should be Spartan.

Palmer while referred to as Commander, her rank is Spartan. Commander is a title, not formal rank.

"You are Spartans now. You stand side by side with your Spartan brothers and sisters. You march into battle together--you do not charge ahead. You do not grab glory for yourself. Spartans don't have rank because [b]Spartan is your rank[/b]. You are naturally superior. We have made you better. You are Spartans now. Start acting like it." 
—Musa-096