I'd keep the M8 if I were you. I have to say the Galaxy S7 is a fabulous phone, but it's expensive for what it is, same goes for the iPhone 6S plus you get locked into an eco-system, but if you have the money those two are super phones.
The LG G4 would be my pick otherwise, but I'm disappointed that it hasn't dropped in price with the arrival of the G5 and there have been some questions around the phone's reliability recently, though I have no idea how serious it is. My enthusiasm for LG has also dampened after my wife's Nexus 5 has been back for repairs twice now after the modem unit first failed and then the speaker and we are waiting for a new replacement phone now.
The M8 is still very competitive against the midrange phones on offer currently. Looked at the Samsung A5 (2016) today as well and it's a beautiful device, but again very noticeable lag...