OnePlus One.
If I was going upgrade it would be to the Panasonic DMC-CM1, and it isn't even a straight-up upgrade- it's the same CPU at a lower clock, less RAM, a smaller screen (this one is very much in the eye of the beholder, the OPO is enormous and the absolute upper limit of how large I would go for a phone) and chunkier than everything else on the market. What it does have is an insanely good camera, using the same camera module as several extremely popular enthusiast-level cameras from Sony (RX10/RX100 lines), and full manual controls and RAW capture all as standard in the camera app. Coupled with specs in line with the current/outgoing generation of flagships and a relatively clean version of Android, it's actually enjoyable to use as a phone too. This is arguably the first device to successfully merge a high end smartphone experience and a camera that most photography nerds will be satisfied using as a primary/secondary shooter. I'd take the extra centimetre of bulk for that sort of combination any day.
There's nothing really in the realm of "regular" smartphones that would tempt me from my OPO though, nothing outdoes it by a significant enough degree to tempt me at all right now.