iPhone 5S. Basically, I used to work in the communication industry, and after trying multiple android devices over that period, I found the iPhone to be the perfect fit for my needs and how I use technology. Of course, I'm kind of in love with the Apple ecosystem anyway, having three different Apple devices I use. That's not to say I'm anti-android (and it's kind of ridiculous that that I have to say that for fear someone will get angry), and there were definitely android features I liked that haven't really been emulated or implemented in the iOS updates. I've just found what works best for me.
I also have an iPhone 5S. I bought it because I needed just any phone, and they gave me a great deal since it was the last generation--the thing works as it's supposed to, and the problems I do have with it I've been able to fix through jailbreaking.
Yeah, I got mine right as the 6 was getting released and it was on sale for a penny. I've also had a 4 and a 4S as well. The only problems I have at the moment are more app-related than software related, such as the app I use for reddit freezing up.