I think the question is somewhat flawed. The real question is, which system is most open and free?

Currently there is no GNU/Linux phone that I know of. Maemo is at best a busybox phone with both closed and open componenents.

From a hardware point of view anythng with Linux is more secure, but most relevant operations today are done with software. That's why the most free and open system is the most secure as well, because it offers you the most options to customise the system, even rewrite the whole thing if you have the ability. Open and free system are also far better at providing specific software for security minded software use, anonymizing and network manipulation. This allows users of such system to naturally use things such as Tor, SSH, Proxy, hardened OS and customised software solutions.

In windows, apple and google you have big corporations who make these things difficult and obscure and opaque systems that make it even more difficult.

No system is safe aside from your own self made Linux distribution and not even that is safe.

Did you all know that Richard Stallman don't surf/download internet sites directly, but instead email these requests to a network of proxy servers that download the content for him and send it back to him per email to open in the Lynx browser?