Not has experience as such with any of them, but just from keeping a finger in the tech world, here is my view.
LG G3 - nice device, don't think LG are to bad with updates speed
Galaxy S6 - obviously 'the' phone to have at the moment BUT, it has a sealed in battery which first reviews says isn't great. You won't make it through a day of moderate use, especially if your working (and playing them tunes). Also Samsung have proven to me with my S4 that they are terribly slow on the updates...
Nexus 6 - nice device and as NJDude stated, being a Nexus, you get the Android OS updates before anyone else.
Galaxy S5 - For me I feel this was a bit of a flop for Samsung. Some features simply don't work, especially the fingerprint scanner. It looks really nice and I was tempted at one time, but unless I were to drop on one dirt cheap I think I would look elsewhere.
So, if you can handle the mid day re-charge and have cash to spare then I would say S6.
Next, the Nexus 6 or LG G3, Both are very nice, although considered by some to be old new now. Can still handle the job though.
Lastly comes the S5. Looks nice and is nice on paper, but doesn't seem to quite hold up to it if you want those additional features, If you don't want those, then it could be a contender