Which iOS upgrade is the best for iphone 4s?

I'm deciding to buy iphone 4s but i have heard that after upgrade to iOS 8, 7 and even 6.1.1, its battery life becomes very short and problem of Wi-fi also. I'm not a heavy phone user and i like the design of iphone 4s. Please tell me the upgrade in which it will work fine. Should i just restrict to iOS 5?

Why not move up to 5C since its free on contract? No one writes software for old OS anymore, so its stupid to go back to that.

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