I would say Ranger school, without a doubt. Like someone said earlier. Battle Staff is a great challenging course, I have never been, but most people I know who have end up in a Staff job once they have the required Squad Leader or PSG time. In the Heavy Infantry world (Bradleys and Tanks) Master Gunner is everything, however you could end of working at Battalion or Brigade as the Master Gunner as you gain rank. I am Ranger qualified, and I can say no matter where you go a Ranger Tab opens up doors. You leave Infantry Branch and fall under Ranger Branch so you get more say so over your career. It's the best Military Leadership school, every branch of service can attend and when you graduate you will feel like "the man" with a wealth of knowledge.
However Ranger school is far from easy. Good luck