I don't even have to think about this, definitely the iPhone 6s Plus.
While I am not a serial upgrader, I did upgrade to the iPhone 6s Plus at launch. Coming from this upgrade order, Original iPhone in 2007, to iPhone 4s at launch, and then the iPhone 5s at launch, my wife cracked her iPhone screen a few months before the new iPhone 6s were to launch, and I told her that I would give her my iPhone 5s when the new iPhones launched.
I actually hated the iPhone 6 design, but decided to give the Plus size 6s a try, thinking that I would like the large screen. I was wrong.
The phone was just too big. I couldn't use it one-handed, had trouble taking photos with it one-handed, couldn't fit it in my clothes' pockets. I couldn't fit it anywhere in some of my cars, which I would end up laying the giant phone on the seat, it would sometimes fly off the seat going around a turn.
Mix that with all the iOS9 bugs, and I just hated my expensive, large iPhone.
My wife ended up cracking the screen of the iPhone 5s I gave her, and this is around the time that the iPhone SE1 came out. I got her the SE1 (and an Otterbox Defender case), and when I was installing the case on her new iPhone, I was a little jealous of the smaller size. So much so that I considered getting one, but I didn't want to downgrade my storage space (128GB on the 6s Plus versus 64GB on the SE1).
As soon as the 128GB SE1 launched, I sold my iPhone 6s Plus to a friend and got the SE1, with no regrets.