I thought Nexus remains the smoothest in the long run due to it being stock?
It absolutely is. I've used Nexus phones sine the N4 (2012) and would never use an Android that wasn't a Nexus. It's the Android version of an iPhone -- no carrier bloat, no vendor bloat. If you've never seen a non-Nexus Android (or really messed with one), the carriers add the most annoying, non-removable apps you can imagine, and so does the manufacturer. In samsung's case, they also add their own launcher (effectively the phone's GUI), called Touchwiz, and it sucks.
Hence why I want to try stock Android with the Nexus, heard a lot of good stuff about pure Android.
It's the only way to Android.
I get that Apple is falling behind, but I tried to run away from the Apple farm playing with Androids and keeping them during the 2 week return windows and couldn't stay with them, running back to Apple. The experience IMO is much smoother with Apple. I love the Samsungs and held a Note 7 in the store and debated getting it over the iPhone 7 plus, but Android always kills it for me. I think it's a mess, despite people claiming they are about as good as iOS or better. Can't hang. I hope you enjoy the Note 7.
Once again, Samsung's hardware is pretty nice, but their implementation of Android is horrible. Yes, this is my opinion, but many Android users share it, and it perfectly explains your reaction.
Having had said that, I don't think Apple's falling behind... I think most people got used to Apple's mobile dominance that it looks that way, but remember, the mobile ecosystem's matured, thus it's less Apple falling behind and more everyone else catching up. The technology's not here yet for the Next Big Thing? so manufacturers are testing all kinda add-ons to see what sticks and what they can monetize to these portable computers we call phones -- you can only add so many things to them. With that, however, we get what appear to be incremental upgrades.
Also, Android handsets compete with each other as well as the iPhone, so as was said before, they have to add all of these doodads. Funny how none of them become a standard until (and if) Google bakes it into the main Android OS, though...