Some of the most confusing ranks to address in the Army are the Warrant Officer, Sergeant Major of the Army, and Master Sergeant. All others are easy enough and you get a great example from people every day but you're here either because you are thinking of joining, new, or had to look up how to address a rank because there are some people around you that make it a bit more confusing than it should be. Let's get to it.
First, understand that all guidance and direction on how to address rank in the Army comes from AR 600-20 Army Command Policy. Policies can change over time so click the link to find the most up to date policy for this question on Army Pubs. The date of the regulation I am referring to here is 24 July 2020.
During my time in, I have have heard three different ranks (or groups) debated. I'm going to cover those first.
1. Sergeant Major of the Army. I've had the SMA visit the installation I was on to hold NCOPD or Q&A sessions a few times. Each time there are a few people that address the SMA when speaking as Sergeant Major of the Army and others that just say Sergeant Major, so which is right? Sergeant Major.
2. Master Sergeant. This one is odd and I have personal experience with confusion here. When I was a private at my first unit, I was adamantly directed to address all MSG personnel as Master Sergeant when speaking to them. This led to a lengthy part of my career addressing all MSG personnel with their full rank. It wasn't until about 6 years in that a MSG finally told me to knock it off and just say Sergeant. Was this just because he was more relaxed? Which is right? Sergeant.
3. Warrant Officers. This group is a bit confusing mainly because they are a mythical creature not found anywhere other than an aircraft, boat, or brigade or higher shop. With the lack of opportunity to give the greeting of the day or interact with such magical beings it can be confusing to know how to address them. I've heard chief, Mr. or Ms., and sir. What do they respond to? All of them. Will they yell at you for saying the wrong one? Not in my experience. When referring to them in the third person usually in a meeting or when getting directions to their location at the end of the rainbow, it's usually Chief. What about when you are talking directly to them? Which is right? Mr./Mrs./Miss/Ms. for a WO1 and Chief/Mr./Mrs./Miss/Ms. for all others.
Hopefully that helps out. Why take my word for it, see below. For any others just follow the link to AR 600-20, check PDF page 14.