There were many great 2013 Android flagships. I think Android got really good by Jelly Bean/Project Butter (2012). We can go back as early as 2012 with Samsung Galaxy S3 (Summer Olympics and Android hero phone) and the Nexus 4 (gamechanger on value).
Quad core Snapdragon SoC (400 or above)
2 GB RAM
Jelly Bean/KitKat
^^^ Some ingredients for a better and longer Android experience. Sony and Samsung flagships can swim now. Camera also improved on some of them. Even a Moto G from last year should run solid for 2 years.
I haven't really been amazed by the 2014 Android flagships except maybe their battery life. Many Android flagships with Snapdragon 801/3000+ mAh started surpassing iPhones in battery life. Sony Xperia Z2, Z3/Z3C, OnePlus One, Oppo Find 7a, Samsung S5, and HTC One M8, etc.
I think we sorta reached our summit with specs. There shouldn't be too much improvement from here on out except for maybe lithium sulphur 2-8x battery life and maybe different form factors like foldable screens. I still see people using old Nokia candybar phones from a decade ago as backups or iPhone 4 as main phones. Phones can last over half a decade if you want to and if it can still hold a decent charge. Except people get sick of tech fast and seem to want something newer much quicker because a 2-3 sec delay is too much time wasted for most spoiled, impatient users.