I haven’t had as many iPhones as many here. I’ll rank them, though (when I used them, excluding posterior developments):
1: iPhone 6s (128 GB, iOS 9.3.3): My favourite iPhone ever. It was speedy, battery life was decent, it has a headphone jack. My second iPhone and the one I’ve used for the longest time. Apple forced it out of iOS 9 to iOS 13 and obliterated it. Currently in a drawer.
2: iPhone 6s (32GB, iOS 10.0): Currently in use as a second phone, mimics the advantages of the other 6s minus storage. Performance and battery life are top-notch even after six years (bought in 2016). It is currently my favourite iPhone, if only because the other 6s is now infinitely worse. In spite of the storage, I can call it my favourite iPhone ever, too.
3: iPhone Xʀ (128GB, iOS 12.3.1): Currently my main iPhone, and in spite of the lacklustre camera features, it’s extremely fast and battery life is astonishing, even three years after purchase. I couldn’t be happier.
4: iPhone 5s (32GB, iOS 8.2): Currently in a drawer, with a cracked screen (unwittingly dropped a power bank on it. Poor little tempered glass couldn’t save it. Not worth repairing for obvious reasons). My first iPhone, very mediocre battery life but I loved it. I was in awe with Touch ID.
5: iPhone 7 Plus (128 GB, iOS 10.2): Traded in to Apple. Came with a broken camera and bad hardware which caused lag and unusable portrait mode, awful camera quality. In spite of that, it was my first iPhone with great battery life (north of 12 hours of screen-on time). Gave it to a family member and used my 6s on iOS 9 until I got the Xʀ.
6: iPhone 5c (8GB, iOS 10.3.3): Currently in use, very occasionally, for music. Unusable, in every aspect. Slow, laggy, bad battery life, awful camera, no storage. Only advantage? It’s so comfortable to carry! Used as a secondary phone until it was replaced by my current 6s on iOS 10.
As a footnote, there is one iPhone I would’ve liked to have: The 8 Plus. I’ve wanted a good plus ever since the 7 Plus came botched from the factory. Unfortunately, I’ve never had one.