K8s has made a lot of progress to be used outside of GCP recently, and might be a fit.
Swarm is still pretty new, and although promising, I wouldn't be super confident yet with the stability at scale,
Mesossphere looks great indeed (had an eye on it for a while, but never played with it)
Since you're already on AWS, ECS is quite good to. It's a new step in a aws-lock-in though. But if you have limited resources (in time/budget), it could definitely be a great fit.
The ultimate best fit for your use-case will depend on your very own requirements: