I am not sure I find Spring an attractive proposition and Ktor seems rather young, slow and not that well documented.
What do you think is the best option?
We have a very small Ktor service in Production, and it works nice, for legacy reasons, we're using http://sparkjava.com/ for the heavy lifting. An alternative would be https://javalin.io/
I would not start with Sparkjava anymore. The way you write handlers is quite okey (compared to other frameworks), but there are issues with how it's connected to Jetty and relies on singletons that will be painful if you would like to do advanced stuff. It's on our todolist to swap Sparkjava with Ktor somewhere down the road.
To be honest, Ktor seems to have come a long way, the docs improved a lot last year and it seems well thought out. I would give it a try. It's quite easy do decouple your application Handlers from the underlying framework via functional composition, so there is no big lock-in Risk.
In my experience, all three Frameworks are way better than the regular Java-like approach with annotating classes. Request-Context specific information ("The user making the request") is very hard to get to this way and it's usually untyped. On top of it, you are locked in HARD to the Framework. Swapping out a Framework that just mounts Functional Handlers is way easier...
EDIT: Ktor does look a lot better today and now that Kotlin coroutines are actually stable, I'd definitely check it out.
Vert.x already did the Kotlin plumbing on their own so you can use coroutines and similar goodies out of the box, Spring Boot supports Kotlin natively AFAIK, and something like Dropwizard should be easy to use as well..
Seriously, don't limit yourself, part of Kotlin's beauty is how easy it is to use it with Java products and reap immediate gains!