Hello Sir,
We have very good experience in developing chat bots using Kik, Facebook. In alexa, we do not have much experience but we have gone through their api and we are able to create chatbot in that as well.
According to me, if you are preparing chatbot for your end user, I suggest to use Facebook or Kik. If your target audience in in US or Canada, then Kik will be your best bet.
Thanks
Shiv Agrawal