iOS 10. The most stable day-one release I've ever used. Other people may disagree but there's about half a billion devices, if not more, running iOS 10. Apple's QA and stability for that many devices is nothing short of stunning.
Plus it's one of the rare Apple iOSs that don't cripple the phone it's installed on (possibly with the exception of the iPhone 4S). It runs beautifully on the iPhone 5 and above. Even iOS 5 running on my iPhone 4 significantly slowed it down. iOS 6 pretty much crippled it and iOS 7 killed it.
Gone are the days where you're having to buy a new phone every year, else you'll live with an intolerable daily driver. Yes there's a lot to do with the significant CPU bumps as of late, but today's iPhones have never been better for longevity.
Have to say that iOS 6 was the most stable , probably because there weren't too many changes from iOS 5. However iOS 10 runs great on the five year old Iphone 5 which is something that can be said for most other versions of iOS which crippled phones two years older.
Only because I use fairly modern iOS devices though. Toward the end of an iDevices life I don't like using the latest OS because its typically rather slow. Back when I was using a 4S I remember greatly preferring iOS 7 compared to 8 purely due to speed.