I'd place that bet on Northern Virginia as guaranteed and Chicago, Atlanta or Dallas as the other most plausible, with Toronto being a slight possibility since they're going with two.
Colorado is nice, but it's nothing compared to the Texas population and economic boom. 28m people in Texas, versus 5m in Colorado. Texas is also building high-speed rail, which will end up linking all of their major cities, an immense bonus. Denver / Colorado also sit in the middle of a dead zone in terms of major populations, along with being slightly too far west from any major ports (Dallas or Austin get you close to the Gulf).
Chicago is far larger than Denver to draw talent from (including more surrounding populations) and far larger to draw general labor for. All have relatively good real-estate prices (comparable to Denver), plenty of tech talent, and are large shipping hubs. All three also have slightly lower tech industry wages than Denver. Toronto's tech and non-tech wages are far lower than all of these. Toronto gets them out of the US, but still very nearby, as a talent immigration hedge (political insanity hedge).
Dallas / Texas gets an interesting nod, because Bezos has deep ties to Texas. He's putting one of them in Northern Virginia because of WaPo, his second home there, and the DC politics. It might make sense that he'd favor Texas for similar self-beneficial reasons.
Atlanta, Dallas and Chicago are more diverse versus Denver, which is likely an important consideration for a mega corporation like Amazon. Chicago and Dallas are dramatically more diverse.
Toronto's extreme real-estate prices, double that of Atlanta and Chicago, is likely a big negative.
I give Denver a zero shot at landing one of the HQ2 sites.