I really like what MS is doing with Windows phones these days for Windows 10. If they ever overcome the app shortfall, I'd probably get a flagship Windows phone.
However, since I like apps, I'd probably go for either a Nexus 6 or Galaxy S6 just for a decent Android phone with good camera and decent app selection.
If I did not have a iPhone I probably would get the Blackberry Passport. I really dont like android and i would want to support a Canadia company. That being said I have no plans to switch my phone. Really good question, we all know we have thought about other phones from time to time
I've had iPhones and Blackberrys during my smartphone years so I would probably go for the Blackbery passport, never really got on with android so I would stay well away from that.