I think we're putting too big an emphasis on airports. Atlanta, Chicago, Dallas, and Philadelphia have bigger/better connected airports and significantly larger populations. I believe Denver is the most expensive non-coastal real estate market in the country. It's definitely more expensive than Atlanta, Chicago, Dallas, and Philadelphia.
I think Denver's transit is just okay. It's better than some other US systems out there, but it's far from great. I doubt we'll ever know a world where RTD is a peer of MTA/CTA/SEPTA/DC Metro. Does anyone know what the timetables are like for Denver's light rail? Do the trains run 10-15 minutes apart?
Dallas would definitely be in my top 5. It's certainly my first out for top 3. From my perspective, Dallas has all the issues Atlanta does. There is an existing unmet demand for housing, transit is effectively nonexistent, and traffic congestion has worsened considerably. Adding 50,000+ employees to the mix would make a poor planning situation worse. I also feel that Dallas/anywhere in TX outside of Austin isn't a great cultural fit for Amazon. This is off topic, but I'm interested to see what course Dallas charts moving forward now that it's facing the affordability issues of more established metros. Two of my closest friends are currently in Dallas and will likely be moving back to Chicago within the next few years. They found themselves in Dallas due to a job transfer, but were open to the idea of raising a family there because they believed the housing was dirt cheap. They've now realized housing is no cheaper in Dallas and they have no support network for when they ultimately do have kids.
Raleigh and Durham are too small. At a minimum, Amazon needs to be in a region that is just as large as Seattle. They'd be wiser to go bigger IMO.