HTC One M8 at the moment. In 2014, I've had the M7 (previous version), Nexus 4, Nexus 5, iPhone 5S and Samsung Galaxy S4.
I've had the M8 at least 4 months now which for me is a long time

However, its proving to be a difficult phone to replace even if my feet are itching.
The Nexus 5 was my second favourite - stock Android is a lovely thing to behold particularly with 5.0 on it, and I never found the camera that bad (laggy mind), and the price was great, however, the storage was a problem (I had 16gb version) even though I tried to use the cloud, more often than not it wasn't an option when I needed it. Battery life was the worst thing though, but really it was just last gen in that respect. My girlfriend still has it!
The iPhone 5S was close behind. Storage was a problem again, and the thing was too damn small, and the battery life was no better than the Nexus, but the sound quality and camera of the thing were far better and it felt (and was) more expensive. A 64Gb iPhone 6 / 6 Plus I have used at work, and they are superb on the brief time I've had.
Which gets me onto the M8. I got mine second hand < £300, and a 64Gb memory card. The camera is not the most detailed but rather underrated IMO, its fast and easy to use. The sound quality is on a par with the iPhone as is the screen (some would argue better, but hard to notice in reality), and the battery life is seriously good (2 days with ordinary use - I charge it every night but normally have around 40 - 50% at the end of a moderate-heavy day and even around 15 - 20% the next morning following a day of hammering it for most of the day. While I prefer the look of stock android, the Sense ROM adds some worthwhile extras and is just as fast in every respect so horses for courses. FM radio, SD card and IR blaster (although the latter has proved useless for me) are worthwhile extras and the build quality is easily the equal of the iPhones.
I tried a Z3 the other day. Nice but no better than the M8 and the camera was relatively disappointing indoors on auto.