There are quite a few good options out there. I've used two and they were both good. If you want to use the same bot for multiple platforms (Facebook, Slack, embedding on a page), I suggest Microsoft's Botbuilder Framework. The only caveat is that the ability to connect to multiple platforms requires you connect to Microsoft's (free) bot framework service.

The Bot Framework SDK comes in several flavors and is fairly robust: https://dev.botframework.com/

Sample code: https://github.com/Microsoft/BotBuilder-Samples