The best android for s4?

If the update is coming from the carrier, eventually you won't have a choice but to go to Lollipop. As a tech support representative, I have not noticed any issues regarding this and the s4 that are very common that come to mind.

 

I have the S4 on KitKat and my carrier won't offer any more support nor Android updates for it which could explain why they sold their remaining inventory for the Cyber Monday sale. I'm perfectly fine with KitKat although I'm one who wants to be up to date. I also, heard Lollipop is great on the Samsung phones so that's highly up to you. Personally, I would upgrade if offered to me as I am too chicken to flash it on my own etc.