iOS 6 was at the time the least inspiring iOS to me, but funny enough it is today what I would call my favourite overall
It was fast, stable, brought some great new features (at the time, I thought turn by turn navigation was worth it alone...once I came around to it) and overall was such a solid OS.
I liked features brought on by older iOS versions such as iOS 5 with the notification centre, weather widget, iMessages, among other things - and iOS 7 bringing a new design and control centre (which was sorely needed at the time but in hindsight probably does not hold up as well as the older "classic and mature" design), and iOS 8 which is my least favourite experience so far due to the bugginess and the problems i had with it at launch, but it has brought some of my favourite features overall such a third party widgets and quick reply text messaging or deleting notifications from lock screen. I use quick reply often and it is probably my most used subtle feature.
iOS 9 beta is showing promise, I really like the subtle San Fran font (much easier to read) and little touches like how the keyboard has capitol/lower case letters now or how the battery usage stats are presented. I find it is also more stable for me in its beta than either iOS 8 or iOS 7 were at similar points in their beta cycle which is really good.