I currently own and enjoy:
iPhone 6+
Nexus 6
Note 4
Nexus 5
iPhone 6
This _partial list of my currently activated models_ is but a few examples of smartphones that are as close to exactly the same smooth, fluid, 100% lag free operation as they come.
Why so many new phones? Simply because I'm a very active smartphone enthusiast who enjoys variety and the ability to "grab & go" with whatever appeals at the moment I'm leaving the house.
I get the luxury of first hand experience with a wide variety without having to rely on others reports and opinions.
The phones listed are all so well engineered that the level of parity when one steps back to look at the big picture, differences in quality, speed, and reliability in real world use is negligible.
The "Best Smartphone" on the list is "The One You Like ".