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

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.