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.

The Air Force is notorious for overusing "Sir" and "Ma'am" for just about any rank and it's extremely annoying to say the least. I never considered myself a hardcore airmen who bleeds blue by any means, but I do think the Air Force should be more formalized in addressing each other by proper ranks and the enlisted corps should not be addressed using titles typically used for officers (e.g., Sir and Ma'am). Having gone through a joint environment tech school in the late 90s, it was obvious the Marines and Army took a lot of pride in being called their specific enlisted rank, whereas, in the Air Force, you are a "sergeant" whether you are E-5 (Staff Sergeant), E-6 (Technical Sergeant), or E-7 (Master Sergeant). It was only at E-8 and E-9 when the titles seem to change to "Senior" or "Chief". I personally took a lot of pride in being enlisted and always preferred being called "Senior" over anything else.

Gunnery Sergeant is proper. Gunny if you know him/her. Guns if you truly know him/her. Sergeant if you don't know better.

To me? No matter what branch, I would address a senior NCO of any branch by full rank. If corrected, I would apologize and refer to them by the correction.

Lower pay grade would be soldier(Army), sailor(Navy), marine(Marines), airman(Air Force)

I think I would stick with Sergeant for Marines and Air Force. Navy always confused me.

Once out of Boot Camp, NCO's and SNCO's were referred by there rank. SSGT Smith or 1stSgt.

Being a 1st class in the navy with 17+ years in an NCO hates being called sir or ma'am. You should study the NCO rankings for all militaries. An E-7 in the navy is a Chief, an E-7 in the army is a sergent first class, an E-7 in the Air Force is a master sergent, and a marine E-7 is a gunnery sergent. I hope this helps.

If they are the same rank or below i call them by their last name. If they are enlisted above me I address them by their rank. If they are officers i call them sir or ma'am unless she is a WO then I call her sir. I still hold to the ranks even if they are retired and I know them. That's my spin up.

well....if one is brave (or stupid...we all know that fine line), just address them like this-
"Yo Dude.....wut up"
LOL

When I first showed up to my unit, I thought I knew rank structure, but then I was introduced to all new titles. At least in a Field Artillery Battery.
You have Gunner, Chief, Gunnery Sgt and Smoke. Those were the ones just in the firing battery.

In the Army, E5 SGT through E7 SFC, you address as Sergeant. E8 Master SGT is addressed as MSG (Master SGT), E9 1SG is First SGT, E9 Sergeant Major is Sergeant Major (SMG) and E9 Command Sergeant Major is Command Sergeant Major (CSM). As for the Marine Corps, unless you're in Recruit Training and are instructed to address your Drill Instructors as Sir or Ma'am, in the fleet you will address them by their Title/Rank and Name "Drill Instructor Staff Sergeant Ledyard, Drill Instructor Sergeant Shelton. Gunnery Sergeant (insert name here), Master Gunnery Sergeant (insert name here). Navy and USAF? no clue.