It is a general question, and the answers are a bit old, but it is my field, so I’d like to contribute with my 2ct of experience. First, we need to define what we mean by “most secure” and what we mean by “Mobile devices.”
I would say that by “cell phone,” we all may agree on a mobile device able to make and receive voice calls, at least.
By most secure, we could mean a cell phone that doesn’t disclose our location and where calls cannot be in tapping.
The U.S. President (and other ten lucky ones) has the most secure phone according to the criteria mentioned above. But I am not 100% sure that nobody knows where he is, just merely considering how cell phone networks work (If I don’t know where a phone is, how can I deliver a call?).
Then we have, probably, the Boeing Blackphone,) which is not commercially available for everyone, but also Sectra Tiger/S. And another one from some France company (I forgot the name), none of them are commercially available.
Some Mobile Devices
Then we have a list of suppliers that mostly use Samsung devices and replace the original software with some customizations made on their own. It is worth to mention that, while Samsung was the most hacked android phone a while ago, several security firms found out that it came with back-doors in its firmware that would allow Samsung to access your phone, regardless of the O.S. running.
Then we have a list of secure smartphones sold both retail and public institutions. The most prominent mention goes to GSMK/Crypto phone (used in Germany) and Silent Circle BlackPhone.
While Silent Circle started first addressing everyone, it is now focussing on corporate customers.
Another mention for the Sirin Lab/Solarin, the US$16,000 that competes with the Vertu with the Silent Circle adds. Sure, gold and diamond usually make your phone more secure.
Now and then pops up a mention for the Turing phone, but it is not yet available (it hasn’t been for the past few years).
All great, BUT we have to remember that, as long as the phone connects to the cellular network, it can’t keep safe your locations and calls. That is because both site and tapping of calls/text messages and internet access are done at the core network level, using its vulnerabilities, it is not on the phone.
And on the “dark market” tapping a phone costs US$50. You can find more on this subject on our website.
Some More Information
Now, while the U.S. President’s phone is rightfully locked down and he cannot surf the internet nor install apps (as everyone with a bit of understanding should expect), all others allow that: surf the web and install apps. Not only, but most probably also, the U.S. telecom apparatus has been modified to protect that phone from external hacking specifically. That is also why there are only 11 around.
So, I don’t see much of a difference between those phones and an iPhone. I like more the iPhone than those bulky alternatives (or crazily expensive).
One thing should be clear. If a phone connects to any cellular or satellite network is exposing itself to track, tapping of calls/messages/internet, with a pin code sent via text message that a hacker can read. This is how the way hacks WhatsApp), credit theft, identity theft, impersonation, and so far and so forth.