I am off contract (mobile share value so being off contract saves me quite a bit of money and I would be buying outright or using Next). I think they're all good phones and I've done my research extensively on them and I have my concerns with each device which I'll list below, maybe that way people can dispel any concerns I have if they're off base and help with their recommendation.
Nexus 5 - battery life, so so camera, not being an "actual" at&t device causes some trouble/headache being provisioned correctly with the IMEI for LTE services etc...
Moto X - processing power, camera quality, screen size, carrier shenanigans (bloatware, bootloader lock, sim lock, delay in updates, etc.), longevity of device (once carrier/OEM stop supporting updates)
Galaxy S5 - limited storage capacity, kitkat and what content can be stored/transferred to SD card, touchwiz, carrier shenanigans, longevity of device
Htc One M8 - so so camera, power button placement, carrier shenanigans, longevity of device, kitkat and content to/from sd card