Ranked in these QuestionsQuestion Ranking
Pro Everything works very well out of the box
It's engineered to be designed. Design means choice. A collection of best options, components and programs chosen by highly experienced curators/designers/engineers are installed by default. No need to customize. It's best by default.
Pro Great selection of high quality and polished apps
Apps must pass tighter guidelines to make it into the app store.
Pro Consistent performance
Unlike Android, iOS is developed to be used on similiar devices and both the hardware and iOS itself are developed by Apple. This means that they are highly adapted to work with each other. Even after the hardware becomes a little outdated, the performance still remains consistent and smooth on several iOS versions.
Pro Personal assistant called Siri
Siri is an intelligent personal assistant and knowledge navigator. Siri lets you use your voice to send messages, schedule meetings, place phone calls, and more.
Pro Easy to learn to use
The iOS user interface is known by millions. It has mostly stayed the same throughout the years.
The main view is a simple grid menu which displays all the installed applications. On the bottom there is an app tray which displays the icons for the most used apps.
The iOS user interface is especially easy for users who want an easy experience and don't want to spend time learning on how to navigate through their phones. Everything is displayed right in the main view and it's easy to find. The fact that it has not changed, at least significantly in the past years has made it possible for it's most devoted users to keep upgrading their iOS devices without having any difficulty to adapt.
Available in select cars, CarPlay is built primarily around the use of Siri voice commands and prompts, providing an "eyes-free" experience where you can respond to incoming calls, dictate text messages, or access your music library from a vehicle.
A trustworthy OS, secure for use with banking apps.
Pro Support for standard sync protocols
Out of the box support for caldav and carddav.
Pro Apps request permissions when needed
Instead of an app asking for all permissions it will need when installed, iOS apps request permission to access certain parts of user's data only when the current action requires it.
Pro Apple pay
Apple pay, an easy and secure payment option.
Pro Longtime support for older smartphones / tablets
iPhones don't become obsolete in a year or so. Most of them get updates for even more than about 4 years.
Con Not very customisable
Other operating systems like Android or Windows Phone (more or less), iOS isn't as customizable.
Con No file managing
Managing files and directories is not possible. Files are in fact attached to applications. It is an oversimplification that can be limiting.
Con Inconveninet price and hardware lock-in
iOS is only available on iPhone, iPad and iPod Touch, in which these device may be more expensive than the competitors.
Con Tied to Apple
No alternative software can be installed unless you keep your phone jailbroken (with the system outdated and probably with security issues).
Con Older iPhone models with poorer batteries might perform slower
iOS delibrately slows the CPU down on older iPhones with worn-out batteries so as to preserve battery life, resulting in possible slower-than-expected performance. This only applies to iPhone 6, iPhone 6S, and iPhone SE running iOS 10.2.1 and above, and iPhone 7 running iOS 11.2 and above.