I think it's impossible to talk about "most secure" with out being more clear who you're trying to protect against.

Rogue apps, hackers, law enforcement and state actors are 4 different attack vectors that jump to mind that all have different mitigations.

We'd also have to talk about how usable you expect the phone to be afterwards; for example if to make it secure we have to disable wifi, mobile-data and apps, is there any point in having a smart phone in the first place?

I, personally, am much more comfortable using an iPhone than any other mobile phone. I have very little trust in Android phones, especially one that has > 0 apps from the Play store installed.

(I'd still be using a Blackberry if they had any decent offerings, however.)