What is the proper way to address NCOs in different branches?

In my day (1956-1980), all noncoms were addressed as "Sergeant" except for the First Sergeants and Sergeants Major. "Top" was the informal address for a First Sergeant and "Sergeant Major" for the Sergeants Major. Command Sergeants Major came later in my career and I believe they, also, were addressed as "Sergeant Major."

Duh??? With respect!

Just to be safe I would address them as Sirs and Mams if you are not sure.

"Gunnery Sergeant" is the proper formal address. If you work directly with the individual, or have established a collegial relationship, or if you are senior in grade, "Gunny" is appropriate.

Just don't call a Sergeant Major "Sergeant". I made that mistake at boot camp once....

Once.

These days, I stick with Soldier, Marine, Sailor, (ch)Airman (the ch only if I know they have a sense of humor), or Coastie. Occasionally, I'll revert back to preDD214 habits but that doesn't happen often.

On a serious note, when unsure, it's always best to ask what the respectful manner is to address the person in which you are addressing. Something along the lines of "My apologies. I'm unfamiliar with (insert branch)'s regulations. What is the proper way to address you as." From my experiences while in, most will generally take such an action as an attempt to respect their rank (as it is) and will be courteous about the situation.

A Senior Chief Petty Officer threw a fit over the phone when I called him chief in Iraq. The fact that he was coming to me for help did not matter. The Navy takes their rank way too seriously. When I am now I get called by my first name everyday. Took me a while to get used to it.

Knowing a veteran of the Marines and the Army, you bet your ass, it is Sergeant, Staff Sgt., Gunny or Gunnery Sergeant, on up. I have seen a gunnery address a SFC in the Army just as that, SFC.

When I got out, in '82, I had nine days to go before I went from E-4 Sr Airman to E-4 Sergeant. From Airman to NCO; same pay grade. My promotion certificate came in the mail to me at home after ETS. Today, my 3 stripes and a silver star is a Sr. Airman. Anyway, back then, I'd have been Airman and then Sergeant. I was in Law Enforcement and while we were on duty, doing the same jobs and backing each other on the same incidents, we would often just call each other by last names. But, in the office and around others, I wouldn't have even considered calling NCOs senior to me by their first OR last name. They were all named "Sergeant". Even the Senior NCOs, Master Sgt and above, were "Sergeant" unless they were E-9 Chief MSgt and then they were Chief or Sergeant. Officers were always called by their rank. When I was in Izmir, Turkey, the Chief of Security Police was a Sr. MSgt but, since he was "the Chief", we called him "Chief".

The proper etiquette, in my opinion, which several tell me is only worth 1 cent, lol, is to approach the individual and immediately apologize for your ignorance. "Excuse me, but I am unfamiliar with the rank insignia of your branch". "What is the proper way to address you"
In this manner you have automatically shown respect, and earned it at the same time.
You have shown it by not addressing them incorrectly. You have earned it by presenting a willingness to learn. I never met a superior that felt disrespected from asking for the benefit of their knowledge.