I have been watching this thread since it started (I read most of the Android threads, but not one for posting much).
I am another who has multiple Android handsets (and one iPhone). I currently have the Samsung Galaxy S4 (retired), Samsung Galaxy S5, HTC One M8 and the Google Nexus 5. I purchase my phones outright so there is no carrier bloat installed.
I just sold off my Samsung Galaxy Note 3 (not sure if that was a good thing yet) as it felt too similar to the S5, and I was not utilizing the S-Pen features much.
I prefer the S5 for the way it feels in my hand, the screen quality and the camera. When the S5 is in my hand, it just feels 'right'. I do not like the bezel on any of the Samsung devices, but that is easily solved with a case. I have made a few system changes to make the phone a little more responsive (not that it is slow anyway).
I like the quality of the finish on the M8. Sense 6 is great, although the device is not as nice to hold as the S5. I feel like the height-width ratio is wrong and the weight is off-putting. Also, I find the S5 screen nicer. The M8 has the best speakers, however, I only listen to music with my headphones, so it doesn't make a lot of difference.
I like the 'pure' (well as pure as we can get), stock Android on the Nexus 5 and the minimal design. The Nexus 5 is FAST. It also feels great in the hand. I didn't like the screen, so I have rooted mine, unlocked the bootloader and running the Franco Kernel so I can calibrate the screen (and it is really nice now). The screen cannot compete with the S5.
If I had to pick just one device, it would be the S5. It is the only handset of the three that has a 'blue tick' from my carrier, and, being in an area that has a poor signal, this is important (I usually only get one bar). The S5 has the clearest call quality, the M8 comes second, and the Nexus 5 comes third.
I went and had a look at the Sony Z2 yesterday, but I did not like the screen compared to the S5.
In all honesty, I only need one phone (but I just can't seem to help myself). I rarely make calls (I receive more than I make), but I do message a lot via What'sApp and Google Hangouts in addition to browsing online.
I am thinking of selling the Nexus 5 as it does not get a lot of use.